I really don't want to post on this, because I don't have time BUT here goes -
DH cycles several miles a day along a busy commuter route. He does not wear a helmet. He has researched it VERY thoroughly and concluded that the case in favour of them is very, very iffy and that in certain circumstances he is MORE likely to be injured with a helmet on. It is to do with risk-compensation and the fact that the helmet adds a considerable amount to the dimensions of your head and therefore makes it more likely that you will hit your head and/or twist your neck in a nasty manner if you fall off.
DD1 has a helmet. She doesn't always wear it. I think she will do when she rides on the road by herself, but I'm not sure yet.
I'm not going to get into a debate, because I don't have time. So if I post and run, please forgive me. I'm sure there is plenty of discussion on the web if you want to look it up.