I've let a few Bandits live, hoping they'll reform their ways. But when you're traveling with a friend, don't expect them to be so merciful.
Also, I've got something a tad off-topic yet a tad on-topic so what the hey...
I've noticed that when you beat up an essential NPC, you'll get an assault bounty, but when they fall to their knees to surrender the bounty is removed. Then they get up, brush it off, and say something along the lines of "Don't do that again, jerk off."
I tried it on Dengier (sp?) from Falkreath's maid. Which I honestly don't know why the hell she's so essential. All she does is walk around saying that she has to go now.
Also tried this on the Librarian at the College. One too many threats from that guy. Figured I'd check if his bite was as big as his bark...
It wasn't.