I'm guessing you're talking about using aoe in pre-50 dungeons? most people spam aoe in late game dungeons.

Problems with aoeing lowlevel dungeons: Several classes have no aoe at that level. Had a tank pull several groups of mobs and told us to aoe. I responded telling him "we are lancers, we don't have an aoe skill"
Next problem is hate generation. PLD sucks before shield oath at getting hate. AOE spells will rip mobs right off of you even with flash spam.
The healer might not have all his skills. Removing damage by focusing down individuals can stop you from having pointless wipes to trash mobs.