(Perhaps we mean something different by "legitimate"? I think I basically consider an opinion to be legitimate if it's plausible-or-better.)
I prefer blondes to brunettes -- that's my opinion.
I prefer chocolate ice cream to strawberry ice cream -- that's my opinion.
You're saying "opinion" when you should be saying "theory" or "hypothesis," both proposals about the true content of reality. An opinion isn't a proposal about reality, it's an expression of personal preference.
> I think I basically consider an opinion to be legitimate if it's plausible-or-better.
But that's not an opinion, that's an observation that may or may not have a connection with objective reality. Opinions by definition are not "legitimate" except in the sense that everyone is entitled to his own. By design, they have nothing to do with reality-testing -- that's reserved to hypotheses, proposals about the content of reality.
Of course it is. By definition, opinions aren't statements about objective reality, they're expressions of personal taste and subjective experience.
The only sense in which an opinion is "legitimate" is the idea that everyone is entitled to his own, never that an opinion might correspond to reality. The latter isn't an opinion, it's a hypothesis.