Any half decent BLM will know the fights, know which aoes have to be avoided, and plan accordingly. Between slidecasting, teleporting and procs, they can still move enough not to tax the healer much more than a regen or medica2 that may very well be ticking anyway. There are times to just sit in aoe, if it's not that hard hitting, and they have their own personal mitigation available as well if need be, but it's their responsibility to know what's going on