I main a MCH and I have many reasons not to AoE on big pulls. I single burst a target If that particlar mob...

- Spawns other mobs (Summoners in Kugane)
- Hits too hard to be in a big pull (i.e. it was in a pack by itself and was pulled into the rest of the remaining packs)
- Does party wide damage or big single damage (Qarn bees)

So on n so forth.