That's not entirely fair. "Weakly held" can mean "based on limited information". E.g. if you are views as credible you can bring to light information that allows the person to change to a more strongly held opinion (that is not "stupid"; yes, your point about conceit does have some merit).
You can also do this sort of thing by providing another way to look at a broadly accepted set of facts.
You're doing a "nice thing" by changing someone's weakly-held opinions to match your own? That seems more than a little bit conceited.