If someone can't establish their credentials by communicating clearly and appropriately, their label means little.

Peer review would encourage what I might call "pack rule," in which some group (say, communists) bands together and wholeheartedly approves any member of the pack.

I don't think that users have in general been mislead by know-nothing comments. In other places, I have seen instances of people with great credentials using them to imply that their opinions were gilt-edged even when they were talking about things not related to their credentials.