That's not a reason for resisting discussions of possible measures of composer quality.

It is, though. If you saw a comment thread discussing possible measures of color quality (i.e. forest green is the best color and should be your favorite), how would you react? I would be concerned. If people think musical preference is like color preference, then any statement about how people should value academic music more sounds like an argument about how people should value orange more.

(I am moderately guilty of this. But my argument is essentially that gardeners should focus on flowers that are pretty in the visual spectrum rather than flowers that are pretty in the ultraviolet spectrum, and that strikes me as superior to staking out a particular part of the visual spectrum.)

It would be like saying that the reason people don't like discussions of ethics is that they resist any measure of ethical behavior other than Theory X. But that's not a reason for downvoting discussion of ethics, it's a reason for arguing for Theory X.

You can perform thought experiments along these lines and I think the results will be similar. If I put together some comments arguing that the Muslim way of treating women like property to be protected is probably better for them than the American way of treating women like sexually liberated people, I expect those comments would not be voted as highly as my normal comments, even if I polished them to the same level of quality.

That is, people often seem to use downvotes as an argument against a position that seems to be beyond the pale, and it's not clear to me that's entirely a bad thing. There are cases where it hurts, but also cases where it helps (instead of getting into a heated political argument, one would just downvote and walk away).