In a nightfall, I'll absolutely use self res unless it's strictly void burn, because being kicked to orbit sucks. Also, unless it's void burn, odds are your nova bomb won't kill any adds that are actually dangerous, and it exposes you to a lot of fire if you're going for a lot of trash kills. It's silly not to use self res in these situations. That said, voidwalker with obsidian mind is the most fun class in the game.