During a recent dialogue my interlocutor shut down discussion with the following statement: "this is my opinion and I'm entitled to my opinion". In other words, he was saying that he was as right as possible without needing to actually prove it. Or, put another way, there we was no need to refute his point because he couldn't be wrong about it - because, it was his opinion.
I understand how opinions can't be challenged when they relate to matters of taste such as "in my opinion chocolate is nicer than strawberry". But when statements relate to matters of fact, such as "vaccinations are effective" or "CO2 warms the atmosphere", I believe there is a truth value that can be uncovered through investigation and dialogue. Simply brushing the issue aside by claiming to have an unchallengeable opinion seems to me to leave the issue completely unsettled.