In my example, you were hasting 15 DD's within a 30 second window, and you weren't in their party.
First, you don't need to be in their party either to do it normally. Second of all, casting it faster is nice, but it won't change a single thing in a zerg (Because, as someone else said, you don't start casting after they pop it). And third, casting it under 30 seconds would require to be both on SCH main and burning celerity+acession, and 2houring for the last stratagem; which is counter-productive.
Casting it normally, or AoE doesn't change any tactic either. I would be just as free to do anything I could do after hastes, had I casted them 1 by 1 or AoE'd. The only real advantage here is that AoE form requires much less mindwork because casting it 3 times is much easier than 15.
That's it, there is no other tactical advantage here, not with our current methods and even on the most extreme of scenarios (A zerg). It wil not break the game, nor allow anything new to happen.
It is better wither to be silent, or to say things of more value than silence. Sooner throw a pearl at hazard than an idle or useless word; and do not say a little in many words, but a great deal in a few.