The last crash I had that involved a trip to the hospital would have been less serious if I had been wearing a helmet. No concussion but four stitches in my forehead and five to put my ear back together. Saying that, a friend of mine had a similar crash while wearing a helmet and he ended up needing his chin stitched back together.
The question of whether I was wearing a helmet or not is actually pretty irrelevant when you consider the other factors in the crash. I had an argument with my girlfriend just before I went to the pub. There was a storm with force 7 winds and I was planning on getting pretty drunk. My girlfriend wanted me to take the bus and I wanted to take the bus but because she was telling me to do it I didn't want to. Honest, I'm not twelve years old.
So, off to the pub battling a head wind all the way. 8 or 9 pints later I managed to fall over while unlocking the bike to set off on the five mile journey home. The journey home was considerably easier since I had a tail wind. Unfortunately I didn't take into account the fact that as I rounded a bend the tail wind would become a side wind so I found myself being pushed off the road. There was a ditch and then a pavement separated from the road. I figured I could ride it out through the ditch and probably could have if it wasn't for the traffic cone that materialised in front of me. Over the bars and ended up using my face as a brake.
So, when people find out that I crashed my bike what do you think the first question was? That's right, it's "Were you wearing a helmet?" Not "how drunk were you?" or "wasn't the weather horrific that night?".
Cyclists and non-cyclists get way too hung up on the helmet thing. In the grand scheme of cycling safety it's such a tiny part of the puzzle it should be number 25 on the list of things we're discussing when compared to factors like driver behaviour, driver attitude, infrastructure, being seen, road position, etc.