This comes up fairly regularly here. I'm for freedom of choice, but I can come at this from a different perspective.
14 months ago, wearing my helmet - because I think it prudent to do so - I crashed my bike on a bit of trail I've ridden a couple of hundred times. I was unconscious for several minutes after landing on my head. My helmet was damaged, but didn't look too bad. I walked off the hill, but had to be told what to do and where to go. Superficially I required the removal of grit from my head and took 6 stitches. Apart from being sore, a bit dizzy and suffering a bit of shock I felt 'normal'.
At home, It soon became apparent that I was exhibiting stroke like symptoms - I tired easily, found noise difficult to deal with and struggled to process conversation in loud places like clubs, pubs,etc...
A month later I left my easy, undemanding bike mechanic job to become a supervisor in a fast moving and dynamic renewable installation company. I had previous done a similar demanding job, but as the demands of the job rose, it became clear that I couldn't handle stress well, had difficulty managing multiple tasks, and couldn't seem to think ahead far enough to plan or spot problems before they arose.
I had a minor brain injury.
My recovery has been slow and steady since. I still struggle with conversation in loud rooms, and I'm still a bit slower on the uptake than I was, (although, if you met me you wouldn't be able to tell any different).
I am lucky. I was wearing a helmet, and landed on my head at the relatively slow speed of 23mph. Even then, I have two scars, a chip out of my skull and over a year later, I'm still not quite right. That's from the lowest possible class of brain injury. I shudder to think what the outcome would have been without the helmet.