[panglott]

IOW, helmets are worthwhile as long as there as zero (or near-zero) marginal impact on cycling.

Of course, there is not zero marginal impact on cycling. Helmet advocates never seem to try to estimate what cost there is to cycling rates.

I suspect that even among committed cyclists, there is a marginal cost in the number and frequency of trips by bicycle to wearing a helmet, and this cost is much greater for more marginal cyclists.

[hyperbovine]

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.)