I never owned a helmet growing up, I rode in what is now Old fort king trail, which is essentially the East side of WTA..I jumped, logs, rocks, hills, into Hillsborough river in every possible combination, and I crashed alot to, and never had a significant injury. Fast forward 15 years..I began mountain biking and one of the first trails of course was Flatwoods, which as I said, was my old stomping ground before, I knew anything about mountain biking. So I felt at home on the terrain, and trails, and thus I DESPISED having to wear a helmet..until on main where there is a fork in the road which joins together, I took the right fork, and there was a low branch, I ducked underneath it, but it was a large 6" diameter branch, with a covering of small twig like branches that totally obscured the large branch within it. I didn't duck low enough, and slammed the top of my helmet into it at about 15 mph...it compressed my neck and the impact dame near put me to sleep, for a few seconds, I had tingles in my toes... I had to stop, when I did, I took my helmet off and looked at it, and there was a 1.5" crushed in hole on top of it an inch deep. I wear a Bell X-Ray, very strong, well built helmet, and it crushed it like egg crate. That could have been my skull with the 1.5" hole crushed into it...not to mention, a direct impact without the helmet would have surely knocked me out cold, and I wouldn't have been able to call or ask for help. I likely would have bled out before help arrived.
I still don't wear my helmet as much as I should, not wearing it, to go to check the mail, or to go to the corner store, but I NEVER ride off road without one period, and never allow anyone to ride with me who doesn't have one. First question I always ask when someone, I never rode with asks to go riding, is if they have a helmet.
When I used to race Motocross, I needed a new helmet, and was going to buy a $50 helmet just to get on the track, and a very wise man gave me some good advice...
"If you have a $10 helmet, you have a $10 head, if you have no helmet your head is free for the taking."