simply put, thats only if the game mechanics allow that, as it is, their are times when trying to do that will get everyone killed, also pug is not good at aoe. pug has one aoe move in its natural set, aoe used outside of the correct job is like 65% potency and not nearly as accurate.
pug however is fairly good at tanking one monster, and doing damage while tanking it.

theirs also the matter of holding hate.

essentially if you are talking about a good party versus party combat, you could build it so you have to sleep every add, or you could build it so you gather and aoe everything, but the key to making an epic fight, is to build a system where neither is always the answer, and sometimes you even do both at once. Designing such a system is not impossible, in fact it aint even that far from what we could have now, and imo that would be way more epic.

Basically epic isnt necessarilly taking one mob down at a time. I dont see why you think that is epic. Building interesting party versus party combat is something many people want to see. going back to sleep kill strat working for every situation is not a step forward.