You are making a point which is orthogonal to mine in that if you are conditioning on somebody cycling then they are by definition not on the margin. But no matter; the distinction is academic. I consider myself a "committed cyclist" and there is absolutely no cost to me of wearing a helmet. None. I feel naked and vulnerable without one, and these days pretty much won't ride sans casque, and that's the extent of it. (The thing about being a member of said group is, you've spent enough time in the saddle to see so many people doing so many stupid things on a bike, and a few of them pay dearly for it.)