I've rag-dolled over the bars twice, once when hitting sand on an outside corner at 25 mph and down into a 5' ditch full of large fallen branches (I got a mild concussion on that one, and then rode 25 miles to the finish), and once at 15 mph onto a sharply turning cement bike path at night (I bruised 3 upper ribs on that one), and both times my helmet hit the ground instead of my scalp, and at a minimum spared me a gushing head wound that would likely have ended either ride.

I still do certain gravel/mtb rides helmet-less, as long as there are no cars and minimal trees/rocks.