Helmets are important in certain contexts. I always wear a helmet when riding my road bike. I'm going between 15-40 mph, and in the higher half of the range, I'd really appreciate the helmet in the crash. If I'm being passed by cars going 40+, I assume I'll also appreciate the helmet if one of them hits me.

In the city, when I know that my max speed will be ~17mph and a cars max speed will be ~25mph, I consider helmets optional.