Should a healer deal damage?

Here is a tricky question: Should a holy spec'd priest in a raid try to deal damage when there isn't anything to heal for the moment?

Personally with my priest I often come out of a 4+ hour raid to Molten Core and haven't dealt a single point of damage. The only exceptions are Onyxia phase 2 (Dots! More dots!) and Ragnaros. But all the other trash mobs and bosses in Molten Core or Zul'Gurub I never even try to deal damage. And I have three reasons for that.

First reason is focus. If I start concentrating on dealing damage, I stop focusing on the health bars of whatever raid members I am assigned to. If I'm healing a main tank I would never even have the time to deal damage anyway. But even if I am healing a group of other players, I need to stay focused, because if some mage or rogue in my group gets aggro, he needs healing *fast*.

Second reason is resources, that is mana and time. If I cast damaging spells, I use mana. And if then suddenly we get an add and the fight turns out to be longer and harder than expected, I might have wished I still had that mana for healing. And both casting spells and using a wand blocks the countdown timer for some seconds. If I'm shooting my wand and somebody I should heal gets hit, it takes a lot longer to stop shooting, and wait until the countdown timer is done and I can cast healing spells again.

Third reason against dealing damage is aggro management. When I just heal I usually never get aggroed. But after having dealt damage to Onyxia in phase 2, I sometimes end up getting summoned by her and ripped into pieces in phase 3. Every dead priest is bad news for the whole raid.

But I argued with other priests who say that it is a healers duty to deal damage when he isn't healing, so as to maximize the damage output of the raid. I'm not sure if the tiny amount of damage a holy priest can deal is really worth it, especially if I would use my spells, which have a lousy damage to mana efficiency. What do you say?