Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

I take your point and I am perhaps a little guilty of being over-confident about the possibility of minor benefit at higher speeds.

Saying that I am absolutely adamant that if (for example) I were about to be hit over the head with a sledgehammer travelling at 25 mph I'd be sensible to put on my helmet. Would you turn one down on the basis it would do absolutely no good or would you stick one on on the bais that it might? (I appreciate that this is a different thing to the rationality of riding with or without a helmet).

You may be adamant, that doesn't make you right (in fact it just makes you look more stupid, you can take that personally if you want).
And your version of Pascal's wager is just as useful in a godless world.