I think some damage has been done. Now there will always be people who, having heard the clips of Wiggo that the BBC chose to air today, will throw the line at me "Wiggo says you should wear a helmet so it must be the sensible thing to do" and that cycling without one is reckless and it's your own fault if you get knocked off your bike (regardless of the nature of any injuries you may or may not sustain) if you were so foolish as to not wear a helmet. I've heard that a lot over the years, most recently with the name used/cited being James Cracknell, when I argue against compulsory helmet wearing for cyclists.
I wear a cycling helmet most if the time, and have done for over 20 years. I think I probably started out with the assumption that using them was the sensible thing to do and that they must have been designed to reliably prevent head injury. It may not have helped that when I was younger I'd seen the results (brain matter on the floor in A&E) of a car versus pedestrian head injury incident at a nearby pedestrian crossing. The same logic that sees riders use those horrible bits of green tarmac that guide you up the inside of large motor vehicles to get to an ASL, I suppose -- they're there so it must be right to use them. Indeed shortly after buying my first one I put a serious dent right in the top of it when I hit the road head first after going head over heels after hitting a pedestrian who'd stepped out in front of me (while for some weird unconscious in-the-moment reason attempting to throw the bike away from me to avoid hitting her with metal). That left me with some interesting facial injuries from the straps and a vague sense of either having avoided a nasty head injury or having missed the opportunity to tuck my head and avoid it hitting the road surface at all. After seeing Boardman crash and slide across the road head first into a stone wall on the 1998 Tour, I still thought that a good fitting helmet was potentially a bit of a life saver. I led my fianceé to the accessories section of the LBS on our return from Ireland and the guys there were happy to sell her a Giro. It wasn't until about 12 or 13 years ago that I started to read more about the topic, mostly thanks to a better Internet connection, and shifted to the belief that it was still probably worth wearing one, but largely in the expectation that it may be of some use in reducing the need for scalp sutures in the event if an off, not for preventing brain injury.
I do wonder, though, if by wearing a helmet I'm sustaining or increasing the belief among members if the public in general that use of a cycling helmet is a must and that they do prevent serious brain injuries.