The consensus (common and scientific) is that protective headwear helps prevent injuries. The scale of protection may be challenged under various conditions and results - however - in all but very very few cases, protective headwear will reduce chance of more serious injury. As a coach, I require that all riders attending a groupride wear helmets. I'm responsible for the safety of the group and this is just one factor in mitigating the risk. The same with organized events, helmet requirements cut down on injuries to the scalp and reduce concussive injuries common in low speed falls. That's good.
As a rider, I also do ride occasionally without a helmet. I assume that risk - the same as riding a bike to begin with - as part of the activity that I do. I believe in choice. And I do think that we should be able to ride a bike, publicly , with lower level of risk primarily due to automobile traffic. frankly, there should be no requirement to wear a helmet commuting or shopping the same as walking.
I taught my child to wear a helmet (as much as possible), yet I never wore a helmet when I was young - or even up to age 25 - only for racing - and even then only for track or criterium. No one did. So, pragmatically, I do ask myself why we do this? But I also have seen many many crashes where head injuries occur - mainly cuts, gouges, mild concussion, fractures and do know that headwear MAY have prevented that injury. I've also seen in a single day (race) where one rider died and another survived - both wearing headwear. The headwear would have done nothing for the rider that died (flipped over a rail and fell 30m into a rock strewn gully), and the other rider it likely saved them, or at least reduced the severity of the injury (flying off a corner and smashing headfirst into a tree) I saw both events firsthand.
On the greater whole, the question is should society regulate at this level? Enforcing safety does save lives. But at what point should we reduce personal regulation? I think that's a tough question. I believe that if enforcing safety saves lives that affect other people in that society - then it's probably a positive regulation. However, penalizing the personal choice is kind of like 2 steps backward and 1 forward. My 2yen, anyway.