Intuitively it makes sense to protect yourself from road danger by any means possible and wearing a cycle helmet is one way to do this. However there is no peer reviewed evidence to prove that helmets can do any more than to prevent head injury to a 12 year old falling directly onto hard ground at a max speed of 7mph [check this]. There is even evidence [Bath Univiersity] to suggest that the effective increase in head size can increase twisting forces and worsen spine injuries. Also there are arguments to be considered around the message helmets give to others that cycling is an innately dangerous activity (which it is not) and that to cycle you need specialist extra equipment, with the consequence of putting people off cycling as an everyday activity. So whilst appropriate to high risk activities such as mountain biking and road racing or for risk unaware children and given the lack of evidence as to just how effective they are, it should be a matter of personal choice as to whether one wears a helmet or not.