It depends heavily on environment, I'd say. In Netherlands, for example, where cycling infrastructure is so well developed with protected separate cycling paths mostly everywhere, having a helmet on will only make a difference on those rare occasions where someone is so damn unlucky and lands head-on after a fall/crash with another bike. So sure, it can save your life if you're involved in such accident, but on the big scale, it hardly makes a significant difference. Another funny and interesting thing (and not limited to cycling only) is risk perception - there's plenty of evidence to suggest that perception of reduced risk when a helmet is worn can prompt bike riders to be more reckless with their own safety (as opposed to those not wearing a helmet). That said, however, while I never even had a though of wearing a helmet while cycling in the Netherlands, I completely abstain from riding a bike in Cambodia and wouldn't ride it on-road in most other countries, depending on the infrastructure and driving culture. I'm not worried about falling down due to my own fault or due to some obstacle on my path (happened before, ended in fucked up knee haha), I do worry about car drivers not being careful and hitting me into a coma though.