I guess, then, the reason I have difficulty understanding your position is that I don't see a sharp distinction between science and philosophy, for standard Quinean reasons. The kind of philosophy that interests me is very much dependent on experiment. For example, my own meta-ethical views consist of a list of factual propositions that are amenable to experiment. (I've started listing them here.)

But of course, a great deal of philosophy is purely analytic, like mathematics. Surely the theorems of various logics are not mere opinion?

As for those (admittedly numerous) synthetic claims for which decent evidence is unavailable, I'm not much interested in them, either. Perhaps this is the subset of philosophical claims you consider to be "opinion"? Even then, I think the word "opinion" is misleading. This class of claims contains many that are either confused and incoherent, or else coherent and factual but probably unknowable.