If injury and fatality rates among cyclists are a reflection of risk-- and I think they are-- them helmets don't help overall. In my cycling lifetime, helmet use has risen from approximately zero percent to something like half of all riders. There has been no statistically significant change in cyclist injury or fatality rates during that transition. Surely helmets provide some measurable physical protection, so I'm forced to conclude that they carry along with them other effects that more or less fully offset this benefit.

I don't wear a helmet when I ride, mostly because I can hear and see things around me better without one. I think that's a tangible safety benefit that functions to avoid, rather than only mitigate, a crash. I'd much rather avoid a crash than survive one.

One of the negative effects of cyclists wearing helmets (that is unrelated to their net protective effect, or lack thereof) is transferring perceived responsibility for cyclists getting hurt and killed by car drivers from the car drivers to the cyclists themselves. I think that's a very bad perception to promote.