I am a surgeon and have cared for hundreds of motorcyclists who would be dead or brain injured without a helmet.
I have never treated a bicyclist where a helmet would have made much of a difference. I have never been in a bicycling accident where a helmet has made any difference.
I ride a motorcycle and would not go a single mile without a full face helmet.
I ride a bicycle and never wear a helmet.
Check out Advocacy and Safety and if really interested read through the hundreds of posts and the various links. My opinion is that helmets are about fashion with respect to cycling and is the only item we wear or ride upon that does not do what it is advertised to do.