I wear one because I got spooked in a crash I had last year. I wasn't going very fast, but I took all the impact directly on my (helmetless) head and ended up with a concussion and spent the night in the ER.

Also, I fractured my skull eight years ago, which took many neurosurgeries to correct, so I figure that my head is probably fragile enough as it is. Is the helmet going to protect me if I get crushed by a garbage truck? Nope. Would it have saved me some pain and hassle in my minor crash? Probably.