The most serious accidents are ones where the helmet is not enough to protect the cyclist. So every e wearing a helmet doesn't really reduce the number of serious injuries, while it does encourage more wreckless riding because of the feeling/assumption of safety.

This could be reflected in insurance rates. Mandatory insurance in most states is liability insurance. If safety features increase risk-taking, then safer cars should be more expensive to insure. The insurance companies employ legions of statisticians.

And people who buy insurance are more likely to drive/act recklessly?

Everyone who drives has to buy insurance (except for those few who are too poor/just don't care/etc), so I'm not sure that's a valid comparison.