On the other hand, what would you think of a martial artist who claimed to have techniques superior to those used by the pros (Bayes versus Science), yet refused to spar publicly (let alone fight) more than a few times a year?

If Luke, Yvain or Eliezer claimed that they were superior at achieving predictionbook status than others and refused to demonstrate then I would see your point. As it happens nothing they have said indicates that they ought to be able to dominate on predictionbook (although I would expect them to be better than average).

I also note that predictionbook represents a lost purpose. If you orient your thinking and what predictions you make according to what will make you most impressive on predicitonbook you will not necessarily think the best thoughts or subject your belief's actual weak points to testing. This means I'd say it is more useful for those whose status is not tied up with their performance.

Upon reconsideration, I now see that I was following a poor strategy of increasing PredictionBook usage. I won't retract my comment, but I probably won't make one like that again.

Thankyou.