Here's my personal experience:

I've crashed several times on a bicycle and once on a motorcycle and always worn a helmet. One bike crash involved me flipping and landing flat on my back, no damage, a couple light scrapes on the helmet. Other crashes involved some road rash and minor damage to the helmet plus a headache for a day.

The motorcycle crash was real bad and messed me up, I had a full face helmet on that had a gash along the side after I touched down after flying 30-40 feet, no headache afterward. I regularly get up to 25mph on my bike and ride the same street where this happened, and am certain the same crash on my bike would get me very messed up though the helmet was greatly reduce my risk of head injury. Overall, the other injuries sustained were brutal and had I not had a helmet on I'd probably have brain damage or worse. The docters were surprised that I wanted the neck brace off the next day as there was no head/neck pain and no damage sustained to that region.

I ride with $20 sale helmet Giro Transfer, it looks good to me and the weight is a non-issue. You don't want to wear a helmet, don't wear a helmet. But I get to say "I told you so" when you crash and donate some blood or body parts to the sausage creature.