I'll stop and offer assistance to motorists and cyclists alike who are on the side of the road due to mechanical break down or a flat tire, even if the only assistance I can offer are the use of a cell phone or moral support. However, there is no moral obligation to do so, I do it just to act like I'm a nice guy.
But someone choosing to ride at night with no lights, especially if they're also wearing dark clothing, is beyond my ability or willingness to help.
The flip-side to this, which I think is pertinent, is I've had motorists stop for me if I'm on the side of the road.