A tank should use cooldowns so he can take in more damage, that's what tanks do.
A healer should heal someone's hp, because that's what they do and that's what the party relies on.
A DPS should deal damage lol. I don't expect them to be using cooldowns to prevent damage. The only thing I expect, is to know the fight and dodge when necessary. Sure I suppose it's nice if a BLM would mitigate damage when applicable from a healer's perspective, but not every class has access to cooldowns like that and I really don't find it necessary. I just heal. If a BLM took a hit, because Manawall wasn't up, I don't yell at them and insist they use cooldowns, nor do I blame defeat on such a thing. I. Heal. Them.