I think this is the most concise counter argument to the healer on heal argument.
- A tank, no matter how badly play, still have to constantly hit buttons aka do something through out the fight.
- A DPS, no matter how badly play, still have to constantly hit buttons aka do something through out the fight.
- So it should also stand to reason that a healer has no excuse to just stand idle when he/she doesn't heal.
Also, even an out-of-rotation hit from a tank or DPS still do SOME damage to the boss, and thus bring the party a tiny bit closer to the goal of downing the boss. An idle healer, or a healer casting heal when there is no damage would contribute absolutely nothing to the goal.