It's just that healing xp will most likely be exploited by afk bots. Put the tank in a spot where he gets aggro'd by ranged enemies but gets backed into a corner, and hide your ppu in a way that the enemies only aggro the tank. The ppu always aims at the tank (afk) and clicks his heal every 30 sec or so automatically. Since the tank (probably) never gets pushed from his spot and doesn't take enough damage to die, this is infinite overnight xp.