http://en.wikipedia.org/wiki/Bicycle_helmet
Although the link is not causal it is observed that the countries with the best cycle safety records (Denmark and the Netherlands) have among the lowest levels of helmet use. Their bicycle safety record is generally attributed to public awareness and understanding of cyclists, education, and to some extent separation from motor traffic.
A further source of contention is the apparent arbitrariness of cycle helmet promotion and/or compulsion. Ordinary cycling is not demonstrably more dangerous than walking or driving[32]
Detailed analysis of hospital admissions data also fails to support the idea that cycling is unusually dangerous: a study in the UK found that the proportion of cyclist injuries which are head injuries is lower than the proportion for pedestrians at 30.0% vs. 30.1%.
helmets are not designed to withstand motor vehicle impacts, but these account for most serious and almost all fatal cyclist injuries
While a helmet may mitigate the effects of a fall or collision, other factors (such as maintenance, road conditions, and driver behaviour) are more important for reducing the chance of such accidents in the first place. In general, the value of bicycle helmets[ has been systematically overstated [29].
Cycle helmet promotion or high levels of use may deter cycling by reinforcing the misconception that bicycling is more dangerous than traveling by passenger car
Cycle helmet use correlates inversely with the level of cycling in a given country. Official zeal for cycle helmets is greatest where cycling is a minority activity.
Ok i'm done. Having said all that, wearing a helmet is your own personal decision unless you decide to ride with insured clubs and/or race.
Generally it also doesn't take into consideration rider experience. The less experienced cyclists are the ones that crash the most or at the risk of getting hit by vehicles.