Back from a Roulette Snowcloak. Tank & 2xDPS new to the game/dungeon. First boss, no prob. Yeti, longest boss fight in FFXIV history. Dragoon was nuking all adds (preventing snowballs), and when we did get Snowballs, he'd kill them or hit them away from boss. A week later it dies, get to last boss, explain the mechanics and... Watch as the other DPS eats 6 Lunar Screams in a row (insta-kill). Dragoon from before is flailing around, so it's mostly me and the tank (and literally so when the Dragoon also eventually dies). The Bard at least threw in some MP restores between Raises, but the two boss fights took longer than a raid.

Would situations like that even persuade some of the 'not my job' people to chip in with DPS? Or would you just wipe and start over because it's not your job? No, as a Healer, it is not my responsibility to DPS, ever - but as Taurus above said, it is rewarding to do so (and I see no reason why anyone wouldn't). I do it any chance I get, regardless.