Well I always wear a helmet and so do my kids, I have seen it personally save my skull a couple of times and seen it help my sons head from being cracked. It makes a lot of sense to me for sure.
That said I didnt start wearing a helmet until late twenties.
About 15 yrs ago I also was first at the scene where a car pulled out and a cyclist ploughed into the side of it. But it was dark, rider had no lights and was full of drink. The blood running out of his head and down the street was not the best end to a friday night that he, or I, had had and I am sure that a helmet would have at least stopped the deep cuts and reduced the impact damage to his head. He was knocked right out, the car wind screen shattered and an impressive V dented in the roof.
He lived and recovered after a while in hospital, that does not prove that a helmet is not worth wearing.
From what I have personally seen and witnessed if you have one wear it, if you dont wear one it is up to you but it does leave your noggin more exposed when something does happen.