So are you suggesting that if someone was mistaken in one line of reasoning or idea, that they are necessarily wrong on all?

If not... what's your point caller? All I can see in your statement is someone trying to avoid challenging an idea they don't like on its merits.

Actually, I'm surprised at how many participants in public discourse today, especially amongst the so called "educated" classes, actually are willing to make such arguments with a straight face and even give them credence. All it is is ad hominem... a logical fallacy. Shameful state of affairs. (...and maybe it was never different and I'm just noticing more now...)

No of course not.

I'm saying that he might not be right all the time, not that he is definitely wrong all of the time. It is possible that his "democracy is fundamentally broken" beliefs are correct, but we shouldn't just say "ooh Aristotle was smart and believed this so its a reasonable claim" and move on.

I think he is wrong about democracy and I bring up other cases where he is wrong so people will think twice about his beliefs about democracy.

It is of course true that arguments should be evaluated on their own merits. However, we rarely have time to do that, and so we trust authoritative thinkers based on their record of producing good reasoning. The commenter who quoted aristotle invoked his authority to lend credibility to the quoted statement. You would have discarded this statement unless it was shown as a quote of airstotle. The next commenter reduced the authority of Aristotle by calling his record of good reasoning into question.

When appeal to authority is invoked, it is perfectly reasonable to question the validity of said authority.