Kinda sounds like a silly question. The problem is, there's a significant number of players that don't understand the answer.
Yes, DPS is obviously important. Not enough AND too much.
I've played DPS for probably, oh, 90% of my WoW time. Tanking and healing these last couple of months have broadened my perspective on those two jobs, and provided some interesting personal insight into the role of DPS. I'm not terribly good at either, comparatively speakin'. But I'm learning some things about both. For example, despite how nonchalant they usually are about it, it's REALLY annoying when DPS pulls off the tank.
I'm not talking about the tank missing the pickup and DPS getting clobbered because of it. I'm not talking about a tank trying an instance they aren't ready for. I'm talking about DPS not paying attention to what's going on, between the tank and the healer particularly, and pulling aggro because of it.
This is where the "you need to hold aggo, you need better gear, dps needs to be as high as possible" QQ crowd get annoying. There's a huge difference between a tank that can't hold aggro and a tank that isn't allowed to hold aggro. Massive difference. Most of the time, it has nothing to do with gear. It always has to do with someone, somewhere not paying attention to what's going on.
With a few very specific exceptions aside, though, the vast majority of these situations result in DPS not knowing what's going on with the tank and the healer. The tank simply may not have the option (cool downs, sudden target priority shift, etc) to be able to put more threat up on your target. Time to back off. The healer may suddenly be faced (due to AoE or some spell killing gimmick) with a party down to quarter health. G'head and pull aggro in that situation. Not fun.
Why DPS Matters
One time, at band camp... um... Gundrak, I mean... I'm in a PUG with a Fury Warrior. He struts in all decked out still in his t5/t6 gear. We have a DK tank, a Mage and I'm DPSing as Arms in Northrend quest greens and blues. On the second pull in, I pull aggro off the tank with a Bladestorm. Just one mob. I shut everything down quickly. Tank re-established aggro with AoE and it was fine. I don't think anyone even noticed. And I started giving it an extra beat or two before I hit Bladestorm. DPS was fine for that instance. Taking it down a notch was well within acceptable range. Easy enough.
My dps, however, is running significantly behind the Fury Warrior. And I'm starting to watch the other Warrior closer and, sure enough, he's pulling at least one mob off the tank on every pull. About 8 pulls in, he eats it. Major face plant from a Whirlwind too early. Pulled all four mobs on himself and the healer barely had time to register the change before Fury boy was dead as a doornail. DK is cool, though. He re-establishes aggro, and me and the mage finish the pull.
So we're waiting for the other warrior to rez and he says, "Damn Friday don't you ever Bladestorm?"
Tempted to pull out Recount and show him how I'd Bladestorm on every single pull, but I resisted. Instead I says, "Of course! :D I just try not to do it too early. Otherwise, I might pull aggro and die. That would be embarrassing."
He kept his epeening mouth shut for the rest of the run.
All players have to understand is why DPS matters, not just that it does matter. The first is obvious: To get those mobs and bosses dead. Without sufficient DPS, your healer will run out of mana. When the healer runs outta mana, the tank runs out of health. Very bad things happen at that point.
The tank will control the mob's basic behavior. Aggro and positioning, being the two main jobs for the tank. But there's another level of control that is solely in the hands of DPS. This is the part where a lot of player's understanding falls apart.
The second reason DPS matters is that only the DPS team has control of the speed of the encounter. Too little is bad, but too much is usually worse. If DPS generates too much threat, either through direct attacks or AoE, they'll pull aggro. This in turn forces the healer to heal harder, increasing the healer's threat and burning through mana. As this is happening, the tank has to attempt to recover control over her aspect. Which means less threat put to general uses, burned cooldowns, and a general loss of control. That compounds the problem because the threat threshold on other targets, from the healer and DPS on the proper target, gets narrower and narrower.
Either the healer is going to pull because he's healing both the tank and a DPSer, or the healer is going to pull because the tank is slowly losing control, or the other DPS are going to pull, forcing more threat onto the healer, and sucking the mana right out.
That's the basic recipe for a huge majority of wipes.
Now, again, I'm not talking about situations where the tank screws up on the pull. Nor am I talking about situation where the tank just can't control the mobs. In both those cases, it's pretty cut and dry. Those kinds of wipes are very apparent. Either the tank is in an instance they aren't leveled/geared for, or they need to pull their head out their plate clad ass.
In my first run in Halls of Lightning, a PuG, we wiped like five times on the first boss. Gah! Three times were because someone wasn't paying attention, got to close to his path and pulled. Sigh. The other two times, though, were all me. I wasn't picking up the two adds he carried with him. It was horribly apparent what was going on. Yah, I felt like a noob. I pulled me head out, nabbed the two adds, and wham he went down smooth. You learn, you adapt.
DPS doing it wrong, though, doesn't have an immediate effect. It causes a cascading effect that ends in a wipe. It's easy, in these cases, to blame the tank or healer. The whiners will either blame the healer for not healing them ("Why were you taking melee damage in the first place?") or the tank for not holding aggro ("Why were you hitting the star when the skull is still up?") But when these situations occur, I guarantee it is NEVER EVER the healer or tank's fault. It's always DPS hit something that they weren't suppose to be hitting either directly or indirectly.
What all DPS (yes, I include myself) has to understand:
The group revolves around the tank/healer dynamic.
YES! And yes, the healer and tank aren't going anywhere without the DPS. And vice versa. The problem isn't teamwork. If the teamwork thing isn't there, no one's going anywhere anyway. That's entirely different post. You can't even get to proper teamwork, though, until DPS understands what's going on between the tank and the healer.
DPS Plate posted this ten commandments for all DPS melee folks. Actually most of it applies to ranged DPS as well. It's been much linked and I'm doing it again here because it's perfect. All of the in combat commandments are directly tied to the healer/tank.
Here's how healer/tank relationship applies to DPS and why:
1. The more damage you take, the more threat the healer generates. This in turn closes the threat gap between the healer and the tank. Believe me, there is nothing more terrifying for a tank than tabbing to a mob and seeing your healer's picture. That's panic time for a tank. I assure you, if building threat to put distance between herself and her healer means letting you, as DPS, eat it... well, we'll rez you after. Yes, a tank knows when you pull aggro, pretty much instantly, and yes, sometimes she'll let you have it if it means keeping her healer alive a few more seconds.
2. A good healer will heal the tank first. That's nothing personal. A good healer knows that if the tank goes down, the fight is done. They also know that many fights will finish successfully with one of the DPS down. Oh, and all those mobs you're not hitting hard? Guess where they're gonna aggro when the tank dies? Yah. So there's a little self preservation in the healer's priorities there.
3. Both of the above tie directly into the innate understanding between healers and tanks: namely, if one dies, the other dies. If the tank and healer die, your survivability as DPS drops dramatically. Thus, it is very much DPS's job to keep the healer and tank alive. See also, ten commandments linked above.
"If the tank dies, it's the healer's fault. If the healer dies, it's the tank's fault. It DPS dies, it's their own damn fault."
That glib saying is humorous, if not entirely accurate, but there a measure of truth to it, as well. DPS has to understand what's going on with the tank and healer. These two jobs are incredibly intense (yes, I can say from experience, way more so than DPS). If DPS is aware what's going on, they can make everything run smoother. Here's some thoughts:
1. Pay attention to the over all group health. If your healer is about to switch to group healing, your tank is going to get less. Compensate by doing everything you can to buy the healer and tank some time. Your recount rank be damned.
2. Pay attention to what your tank is hitting. When you switch targets, you should know at a glance how hard you can hit without pulling aggro. Your tank/healer don't give a f@#$ about your DPS ranking. To them, dead DPS = zero DPS. Your job is to kill stuffs. Their job is to keep you alive long enough to do it.
3. Know what your DPS is before you go into the dungeon. Then turn your DPS meter off. (Recount, for example, keeps working in the background, even if it's not visible.) There's nothing more annoying to a tank or a healer than someone popping a DPS meter into /p after a wipe. Fat lot of good that giant DPS did, eh? More important, you can analyze your DPS after the run. Then you'll know better where to make adjustment to your rotations for next time. If you're analyzing your DPS performance in combat, YDIW!
4. Related... your tank (and probably your healer) will know what your DPS is. If it's not enough, they'll letcha know. If it's too much, they'll letcha know that too. Don't worry about it. Get in there and kill stuff as fast as possible WHILE keeping yourself and everyone else alive. Your threat meter is WAY more important than your DPS meter. Always. No exceptions.
5. Know when to back off. Know when to pour it on. Slap the right gear on a blind monkey and he'll hit high on the DPS charts. Yah, I'm looking at you DKs. :P Which, again, is solely dependent on threat on target. Turn off the DPS meter, turn on the Threat meter.
Good DPS kills stuffs. Awesome DPS kills stuff properly. Being awesome DPS revolves entirely around understanding that the tank/healer relationship is more than just topping off her health bar and keeping the bad guys off him. It's for another post, but there's a lot more going on between a really excellent healer and an awesome tank. DPS is an integral part of that relationship.
The best DPS I've ever seen are the ones you barely notice unless you're looking for it. Yah, they're scoring high on the charts. That Frost Mage that can in combat cc at just the right moment to mitigate the spell damage coming in on the tank. The Arms Warrior that can hamstring the right mob in an emergency. The Rogue that peals off to keep sap up. The DK that Death Grips that wayward caster who's targeting the healer, pulls the mob to the tank and then backs off enough that the tank instantly pulls aggro. The Hunter or Lock who can off tank effectively with their pet/minion when everything goes to hell. All of which hurts their raw DPS number.
Those are Awesome DPSers.
They do it without a lot of recognition. Be that DPSer, anyway. And tanks and healers should notice and appreciate it more vocally. I always personally thank my healer or tank at the end of a run. I've made it a point lately to thank those DPSers, regardless of where they are on the DPS chart, who are doing it right because they understand the entire group dynamic, not just the "hold 'em still, heal, and kill 'em fast" part.
It is appreciated. :)