The short version is that for most types of city bike use, a helmet doesn't seem to confer any noticeable additional safety. The analysis is difficult in part because while yes, a helmet can reduce the severity of head injuries, the full cost analysis is difficult. Fewer people bike if they have to wear a helmet, and don't forget that the helmet itself costs money, and must be replaced every few years.
I personally think most people wear bike helmets as a talisman, given the number of improperly worn helmets I've seen. I also think that people don't know if/when it's supposed to be replaced.
