Maybe, being exceptionally generous, you could posit that any given individual only is informed enough about some small subset of things that they hold opinions about. But in my experience, "believing you should have good reasons for the beliefs you hold" is a pretty all-or-nothing affair. Some people will have well-thought out justifications for pretty much any belief they hold, more will have no justification for any belief the hold. It's pretty well-correlated with intelligence, which is backed up by studies that have found that more intelligent people are better at coming up with justifications for their actions. It's not unreasonable to assume that carries over to beliefs.
The core takeaway is to ignore the drooling masses that disagree with you (or agree with you!). Their beliefs have no correlation with truth, and thus do not provide additional evidence for nor against a given belief. You should only adjust your priors based on interactions with people who actually know what they're talking about.