Yeah, except mages aren't built to be just healers.

Anyone playing Conjurer or Thaumaturge and is just sitting there healing is a waste of a party member. If you are spam curing rather than being a productive member of your party, you are not playing to your potential. You have the highest damage output over time of any job in the game. Profundity & Spirit Dart is typically just as good as any other melee attack if not better. You're arguing for more complexity but you're perhaps playing too simply.

Someday a monster may come along that requires 1500 MP worth of healing, but right now there is no such thing.

There is no "Healer" class.