(1) You have no idea of the tank's status.

Do you know what cooldowns the tank have used? Do you know how much TP he has? Do you know how much MP he has? Let's be honest. Have you, ever, checked a PLD/GLA's MP before you as melee dps pulled the next pack because the tank isn't fast enough?

(2) Mess up skill order and positioning.

There's a reason tanks do the run-in-shield-lob-flash sequence. The shield lob solidifies single target enmity on the kill priority so you melee damage dealers could open fire. We all know how many LNC/DRGs even wait half a sec before throwing their lance. Then the Flash establishes base enmity on all targets so whatever the healer does, it won't kill them.

By pulling as melee DD, everything will now converge on you. The tank now must use his first GCD on Flash or Overpower. This means not only do you have a headstart, the kill target also didn't get a shield lob, meaning the tank has less than 50% enmity of what he would have had, which in turns forces the tank to play catch up to you.

Does that 1 GCD matter? Yes it does, in many situations it does. And no it won't, in many situations it won't. But you as melee DD is in no position to decide, nor are you capable of deciding, whether this is one situation where it doesn't matter. As melee DD, have you ever, during the whole dungeon, paid attention to exactly how much enmity each party member has on each target? You probably don't, but the tank does. It's the tank's job to know.

Positioning is less of a problem in today's easy mode content, but in hard, challenging content, positioning matters. When the tank has to move mobs, he often cannot keep up the enmity output at the same time---or he has to move slowly in stages and it may take 3-4 times longer to position mobs properly.

(3) Provoke has a 40sec recast, and is single target.

Taunts in this game is not a button to be mashed. It's an oh-no button. We're not playing with a taunt that has 6 seconds cool-down. We have to wait 40 seconds.

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With all that said, there's nothing inherently wrong about someone other than the tank pulling. As long as:
  1. You are just pulling. I.e., after you pull, you wait for the tank to pick the mobs up. You don't pop cooldowns and unload hell.
  2. You pull properly.
  3. You are aware of the tank's status. Or you are friends with the tank.
Most of the times, number 3 won't be true.

Seriously, if you really can't stand slow people, either put together a party yourself via the party finder, or play a tank yourself. Players are people, too, not some NPC the system spawned so you can have your way all time all the way.