Some people who fly light aircraft for a living (RAF, Police and Ambulance pilots) wear helmets and they are much more highly trained with more sophisticated aircraft than us and so are probably less likely to end up in a field due to an engine failure or upside-down due to a very dodgy landing. Certainly fireproof clothing and a helmet has to help.
I suppose the same arguments were had about helmets for motor-bikes many years ago and more recently for bicycles.