Both his presumption and assumption are wrong. Is it better to assume that everyone commenting is always foolish and wrong, or that everyone commenting is expert and right? Neither assumption is correct.
I have my own set of assumptions about the character of commenters, assumptions which are usually influenced by the site I'm reading, but even when I go into a thread with the assumption that there will be a bunch of people spouting off with an air of authority on some subject of which they actually know very little, I still find that sometimes their comments will influence me. It's a difficult situation. It's the punditry problem really - a pundit can declare some nonsensical shit to be factual and the honest-to-god truth, and you're basically left with three options: first, you can believe them because they certainly seem to know what they're talking about and they wouldn't lie outright, would they? Second, you can disbelieve them but always have this small lingering doubt floating around in the back of your head. A suspicion that maybe there was a nugget of truth in there. Third, you can spend hours fact checking the claim in order to eventually, finally, reassure yourself that yes, they are lying sacks of shit and no part of what they said was representative of the truth.
How often do you actually take the third option? How often can you, really? That's like asking someone how many EULAs they read.