''Ah, but you see this is just my opinion and opinions can never be wrong because they're subjective.''
This is generally true; opinions aren't right or wrong, as they're subjective. I suppose you could say all opinions are right. However, it ceases to be true when opinions are built upon false information or false interpretations of certain (possibly vague) things.
Let's say somebody said: ''Oh no! Dante said 'I had my reasons for helping', so that means he helped because he only cared about his own motives. He only had personal reasons for helping! He doesn't care about anyone other than himself. Therefore, in my opinion, Dante is an asshole and lacks compassion.'' That would arguably be a misinterpretation of the line 'I had my reasons for helping'. Or at the very least, it would be conjecture. The fact is, that line could mean anything from 'I was planning to help anyway (possibly gladly, with one or more things)' to 'I had personal reasons for helping that aren't related to your problem'. It usually means you don't have the time to explain or don't wish to - whether that's because it's embarrassing or time-consuming is not known.
So just calling something your opinion does not exempt it from being incorrect. You just need to make sure it's a sound opinion, instead of a half-baked one. If that prerequisite has been met, your opinion cannot be wrong. Your opinion is not worth more than others'. And so I would advise some forum members to cease acting like their opinion is the only correct one and making aggravating threads about it.