MP for healers is how they're balanced. You are supposed to weigh the cost of AOE vs the MP you'll need to heal your group and your MP management cooldowns. Healers are the only role that can use Piety, a MP management stat. It's their whole gimmick.

MP for caster DPS is a limitation on raises OR core to their rotation (BLM). Not to mention that other casters have at least a tiny bit of decision making during the AOE rotation, where as MP neutral/free Holy Spam would not.