I wear a bike helmet when I ride. I do this for two reasons.
1. It's a place to anchor my rear-view mirror. Being able to see what is coming up behind me makes me safer.
2. If I am hit and killed, and I am not wearing a helmet, the press will inevitably say "Chafe City, who was not wearing her helmet at the time, was struck by a horde of angry cars driven by drunken frat boys. Ms. City was cited for not wearing protective headgear." I don't want to give them the satisfaction.