Well in my experience I've fallen off my bike both with and without helmet and hit my head. Both of them hurt like hell, but only one of them has left me with a dent in the noggin which can still be observed in certain lights. No prizes for guessing which!
To be honest in terms of the image, seeing all these cyclists whizzing round London wearing pro kit with their clip in shoes, lycra, bum-pads, mini-computers and so on, I'm actually inclined to think that the cycling "Uniform" must attract at least as many people as it puts off! People seem to love dressing up - it's all about the best kit and the shiniest new bike. And also perhaps a little bit about the cycling :P A cool helmet is part of the gear.