Although I have little doubt that a helmet can provide invaluable protection in the case of an accident, it seems to be a peculiarly British obsession within Europe.
Having recently returned from Belgium, and having ridden extensively in Germany and the Netherlands, my observations are that comparatively few cyclists in those countries wear helmets.
Yes, I know that they are better served with cycle tracks and cycling is more mainstream; but there even cyclists on the roads appear to frequently shun helmets, whilst in this country whole families on tame dedicated off-road cycling tracks, with minimal risk,such as the Forest of Dean family trail (where i was last Sunday) appear to routinely wear helmets.
I suspect that the H & S obsession in this country, and the omnipresent American influence are big factors in this.
Personally I rarely wear a helmet, purely because I think it detracts from the sense of freedom that cycling offers. I also seem to have noticed a trend away from helmets amongst urban cyclists recently.