Here's my entire thinking on bike helmets: You should probably wear one. But you don't have to. Don't let not having a helmet—or fear of a crotchety old man yelling at you for not having a helmet—keep you from riding your bike. There are plenty of good reasons to wear a helmet (brains!), but also some solid rational arguments against them.
Biking with a helmet is definitely safer than biking without a helmet, but I bet driving with a helmet is safer than driving without a helmet, too. So is crossing 82nd Avenue wearing a helmet, riding the bus wearing a helmet, and playing basketball wearing a helmet. It's a question of: Where do you draw the line between being safe and being paranoid?
For me, I'd say I wear a helmet 80 percent of the time when I bike (hey, I'm average!). Usually when I'm not wearing a helmet, it's because I can't manage to get out the door with keys, lights, lock, helmet, and pants but often it's just because I'm only biking a short distance on quiet neighborhood streets and would rather not worry about it. It's no big deal; nothing worth shouting about. The end.