Doing DPS to the mobs is mitigating future damage. This is true in all content.
Serious 8 man content where the healers do not DPS is failed content, because you will not beat the enrage timer otherwise until you've built up strong enough gear to outmuscle it. No heal spell is strong enough to recover from EX, Savage, or Ultimate enrage.
Everyone's job includes hurting the enemies, and the healers have excellent tools for this.