When I was in the Central Park Skate Patrol during the mid-1990's, we interacted regularly with the paramedics who were assigned to the park on the weekend. It's a big issue in Central Park, since it has a big 6 mile road loop that is closed to car traffic on the weekend, and so then it is used extensively by recreational rollerbladers, cyclists, runners and walkers. There are lots of accidents. The EMT's I spoke to were overwhelmingly in favor of helmets, both for skaters and cyclists, and they had many horror stories about people who had crashed without helmets, vs people who had. I have also read several personal accounts from people who have crashed, and are absolutely certain that had they not been wearing the helmet, then their injuries would have been much, much worse.
I've always thought it odd that people would have such strong feelings about bicycle helmets. It doesn't seem like an emotion that would arise naturally. I have a theory that some people pushed too hard for mandatory helmet regulations, perhaps particularly over in Europe, and this resulted in a push-back and politicising of the issue, much like what has happened with the climate change debate. These days we all know it is possible to come up with studies that say just about whatever you want them to say, depending on how questions are asked, what populations and data are included, what statistical analysis methodology is used, how external factors are taken into account etc. This unfortunately muddies the water, since the pool of available data gets polluted by those who have a chip on their shoulder. Then, as laymen, it becomes increasingly difficult to tell the real, valid science from the faux, biased studies.
Once people become this polarized, reasonable discussion becomes much harder. My personal opinion is that "of course" helmets help to protect your head, and it's definitely not the case that accidents where the helmet makes a difference are "exceedingly rare". But unfortunately, like I said, you can cherry pick the numbers to "prove" whatever you like.
My own feeling is that it should be left up to the individual whether they want to wear the helmet or not. This is not just a libertarian, "stay out of my business" viewpoint (I'm not really libertarian), but rather a pragmatic admission that we have seen what happens when we try to force people to wear them, particularly for something as apparently casual as bicycling (as opposed to motorcycles, where people seem much more ready to accept the mandatory helmet laws). If mandatory laws make people so riled up, then those laws are obviously doing more harm than good, if they make people swing so far over into completely the opposite direction, to the point where they are actively and rabidly against bicycle helmets. Once it's been politicised, the battle is already lost. You won't change minds, since people on either side can now point to "studies" that support their own viewpoint.
Just let people choose for themselves, take the heat out of the whole thing, and maybe people will stop having such strong opinions about bicycle helmets.