I don't feel right without a helmet. Then again, I'm the big puss that wears shin guards too.
True story: about 4 years ago a buddy of mine was riding up the road at Skyline Drive (this is the semi-paved road that goes from the top lot down to the lake) and he passed a man and woman were riding downhill, no helmets. Greg continued up to the top lot, then came down the yellow trail and saw them on the side of the road as he popped off the singletrack back onto the road. The woman was hysterical. "My boyfriend, he crashed!" So, Greg runs up and sees a guy with, let's just say major head trauma, and almost gets woozy from the sight. He calls 911 and they get an Oakland paramedic up there asap.
Apparently, the two were racing and they got their wheels caught up. A tumble later, he's off the bike and into woods. Head on rock or tree, don't remember actually.
There is now one of those memorials you see on the roadside, but on that road. This guy did not make it. I have not been down the road in a while but its on the right side about 3/4 of the way down. I used to see it all the time.
Would a helmet have helped? I dunno... but it wouldn't have hurt.