If you are waiting for the mob to get to you, and this is your logic, then there is a ceiling to your gear helping. This doesn't apply to only haste effects. If you DA even on the first swing, and that ends up saving a full swing, but the bard/etc can't pull fast enough, so you end up waiting once the mob is dead, what did that DA do for you? absolutely nothing. What does some extra attack do for you? absolutely nothing.
2% DA is not going to affect your kill time on most mobs. In fact, on most mobs it won't even proc due to that. And when it does, it can still proc on the killing blow, and not give you your DA at all.
Also, in all of those events, the enemy may have readied a WS right as you killed it, and without said dmg increase, the attack would have gotten off (let's say feather tickle), which makes you kill the next mob that much slower.
There are 2 ways to look at this. Option 1 is the puller can't pull fast enough - no stats matter past killing stuff at the puller's speed. Solution: new puller.
Option 2 is that there is a X second delay between killing 1 mob and moving to the next. If you have 3 DD, and each one gets 1/3rd the KBs, doing it 3% faster, so you can start the X second delay before reengaging that much sooner will add up as much in the long run as the math implies that it will.
Your argument negates any and every stat. Any stat can "overkill", or be useless due to a mob dying.
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