This is actually a very clever nerf to the "spam heal spell" strategy that plagues a lot of RPGs. I assume healing is triggered when you press "Cure" on the screen, and not fully automated.That logic for heal spells doesn't work in practice. It's the same as Ogre Battle used, and that's the one area where the AI failed miserably.
Say you have two party members:
Priest 181/183 HP
Knight 200/350 HP
Who gets the heal? Not the person that actually needs it.
What this will mean is that the best course of action is to not heal this turn, and wait until one gets damaged that will want it more. The Knight is probably packing Def, and can afford to wait to get healed anyway.