My AoE rule is: if there are more than 4 mobs I AoE. If not, I find it's more efficient for both dps to focus one mob down at a time because that way they will die faster than if I run my self dry of tp trying to AoE 3 mobs, using invigorate, and then not having invigorate for the next set of 3 or 4 mobs. Usually the tank will do a big pull followed by another big pull. I've already used invigorate on the first big pull and now i'm at 0 tp on the 2nd big pull, doing a single target rotation slowly cause i'm waiting for tp to regen back to 60 for every skill.

I'd honestly rather SE remove AoE skills from the game rather than them making it so AoE have drop offs. Not all AoE have drop offs mind you. Just the mage ones and maybe SAM it think? Maybe SE was trying to discourage huge pulls, but people will always try to speed run stuff they have to do over and over so they may as well just let all AoE do their full damage to all the mobs. just lower the potency instead of drop offs.