I have a friend who rides a recumbent trike. He never wears a helmet. His opinion is that he's so low to the ground that a fall wouldn't be very far (and his trike is quite stable). When I asked about a car hitting him, he said "if a car hits me, a helmet won't help me much when I'm barely above their tire height- I'll more than likely be dead anyway." He does occasionally get yelled at to put a helmet on anyway.
I can see his point. There is an old man in my neighborhood that rides his bike around without a helmet. He's never going more than 5-10 mph and has the best time tootling around. I'd like to see him wear a helmet, but it's his choice.
I'll always wear a helmet. After taking a spill at almost 20mph and going down head first, I know how important it is to protect my noggin. But it's still my choice in the end.

What makes me want to yell at bikers more than anything is the ones who ride on the wrong side of the road. I want to tell them to get on the other side, because I fear for their safety. I never do, but I want to at least educate them.