I understand the critique. In my view, opinions can be had about matters of fact. (Actually, I'd say that is all we can have.) Some of the underlying facts may be verifiable, some may not. Some facts may be overturned by new data. That global warming is largely due to human activity is an opinion on a matter of fact that is less verifiable than the boiling point of water. In short, some opinions have the advantage of their quality being put to a reproducable test. As for facts, their ability to be proven is rooted in physical reality. Historical facts suffer in this regard, as they rely upon the evidence left, and as they can be contextual, they don't prop up opinion as well as physical ones. BTW, you can substitute belief for opinion here IMO, which better clarifies my view. I think they are very similar in meaning, although opinion is often used for matters of preference too. But I am a relativist.