I know that I can heal myself with food and potions, but I use them up pretty quickly and are forced to wait 24hours for new ones to become available in the stores. I'm not going to pay diamonds to get more food.
Making the healer able to heal the fighter would not make food/potions useless, they are convenient to carry and heal with quickly, during a multi-stage dungeon. I wouldn't want to go back to a healer EVERY time I need healing, but he should be able to heal me when I either 1)choose not to use food or 2)have run out of food and don't want to wait.
You can't play the 'use food' card when right now, the healer is not only able to resurrect the Mercs, he is also able to heal them. If he can heal the mercs, he can heal me.