Red Mage says hello. I've done it when a healer disconnected or died in stuff where I had Vercure but not Verraise. They're not awesome healers, but you'd be surprised just how much content a RDM can heal through if the group isn't taking much avoidable damage. Had a bitchy healer in one levelling roulette group deliberately get locked out of a boss to "teach us a lesson", although when I healed it just fine on RDM the lesson we learned was "we don't need you." They were promptly kicked and we just kept going with three of us until a replacement came in.
Hell, I've healed bosses as PLD on a healer disconnect. Once you get the trait for half MP Clemency, you can put out enough healing for a lot of bosses in 4 person content (especially if you've got Refresh or Mana Shift assistance).
Both cases are just someone using their abilities to try and help the group clear the content. Which is exactly the same logic that says a healer who isn't busy healing should be contributing DPS. Everyone should do what they can to help the group with the tools they have available.