[falcor84]

Well, obviously if we have a better prior, then that's better. But assuming no other knowledge, and especially if we think that other people's priors could be intentionally misleading, this rule seems to offer the best estimate.

[jonahx]

Generally speaking, you never truly have "no prior knowledge". Some relevant past experience, or "common sense", or something tips you away from "all probabilities are equally likely". I think this rule is rarely a best estimate.