I am at the point where it feels as odd to not wear a helmet as it does to not wear a seatbelt.
I figure a helmet won't help in many situations but I don't want to end up in a coma for one of the situations it would have helped in. I figure the scenarios where it would make things worse are probably fairly rare. Besides, I have no hair for the wind to go whipping through anyway, so why go helmet-less.