I agree that "experts say" shouldn't all the sudden become a good argument just because an "expert" says something we agree with, but the reason JP was such a threat (or at least perceived as one) is precisely because he undermined their "experts say" crap by being an expert and saying something else. Heresy is far more threatening to the orthodoxy when it comes from a member of the priestly class instead of some pleb. So it is relevant in certain contexts.