According to New Scientist, an Australian academic has determined (using a mathematical model) that the costs of mandatory bike helmet laws may outweigh the benefits. This relies on the notion that fewer people cycle if forced to wear a helmet, and so do not receive the health benefits of cycling. However, there is some debate about the numbers used in the model.
As a cyclist, the helmet requirement has never entered my mind as an inconvenience. It’s just something you do, like putting on a seat belt.
I imagine some people might be put off cycling in the short term, when helmet laws are introduced, because they can’t be bothered to go out and buy a helmet. However, I can’t really imagine that these laws would reduce number of people on bikes in the long run. For anyone considering purchasing a bike once the laws are in place, a helmet is not an onerous requirement. (Bike accessories are often thrown in for free, at least around here.)