Hmm...you raise some fair points. I guess the rationalisation is that there is a tradeoff between my enjoyment of cycling (of which going downhill faster than 12mph is a significant component) and the inherent risk in doing so.
The issue really in this case is whether wearing a helmet reduces my risk of serious injury when whizzing downhill by 0%, 1%, 99% or whatever. If it's 1% then your argument is quite reasonable but were that it was, let's say, 10% then I think the balance of the argument changes
Full motorcycle helmet up the Snake Pass on a warm day = 100% chance of heatstroke so I'll give that one a swerve if you don't mind.
So long as you have looked at the evidence and made an informed choice then whichever answer suits you is fine.
The reason why I asked this was that it shows a dichotomy
The pro-helmet lobby tends to feel able to assess their risk and take decisions on that assessment... this is deemed acceptable
Yet someone making a similar assessment and deciding that in their case a helmet is not required is unacceptable and that person is "stupid" or a "nugget"