I wear a helmet based on how likely I think I am to go down at speed or how isolated I will be.

I train between 5 and 6 am in a secluded park -- it might be two hours before someone found the body, so I wear a helmet. I also wear one for group road rides and when I'm riding on a path and encountering on-coming high speed bike traffic.

I don't wear one when cruising and running errands on my hybrid. Come to think of it, I see a helmet as part of the overall road kit and generally don't wear it when riding in sandals and street clothes. The exception is when I'm riding with my kids -- I figure if I'm going to tell them to wear a helmet, I need to wear one too.

As far as whether other people should wear helmets, my attitude is whatever it takes to get them on a bike, that's what they should do. Sometimes, helmets increase ridership. My neighbor across the street, a former mountain biker with plenty of crashes in her past had stopped riding because, as a new mother, she was concerned about the risk. Getting her a helmet was what prompted her to start riding again.

Some other people would not ride if they were forced to wear a helmet. I don't think trying to talk them into wearing a helmet is very constructive. Nobody likes being told what to do. I understand that sometimes helmet zealots actually will approach and tell strangers they should be wearing a helmet -- I'd be offended.