They can be more or less useful, though.
According to what criterion? You'd end up comparing a prior to the prior you hold, with the "best" prior for you just being the same as yours. Like with preference. Clearly not the concept Unknowns was assuming -- you don't need luck to satisfy a tautology.
This is our monthly thread for collecting these little gems and pearls of wisdom, rationality-related quotes you've seen recently, or had stored in your quotesfile for ages, and which might be handy to link to in one of our discussions.