I was biking down a hill about 10 years ago. Hit a bump or curve or something - either way, I flew off the bike about 5 feet forward and hit the ground directly head first almost as if I were diving into a pool of paved road.

The helmet took all the impact and my head was perfectly fine (my knees then hit the ground and got scuffed up a bit). Had I not been wearing it...

So yes - please wear a helmet.