Every opinion is relevant and everyone has the right to one. No opinion is unfounded. Saying so basically is saying that the person doesn't have a right to that opinion or even worse.
An opinion might not be based on data, that doesn't mean it is unfounded.
I guess we differ on the meaning of the word, but one definition is "not supported by facts."
If there's a card deck with 9 red cards and one black one, I can have an opinion that a card drawn from it will probably be red, and that's supported by data.
If I'm playing roulette in a casino (and the game is not rigged) and have a hunch that the next number will be 00, that's an unfounded opinion.
Note that in both cases it's just an opinion, since it can't be proven. But in one case there's data behind the opinion, and in the other case there's not.
I agree with you 100% that people are always entitled to their opinions.
And in general I will say that I like the firm hand that mods on this board take.