errors that aren't really there
The irony of that bit in this topic amused me.

(EDIT: And wow this was a long post. Oh well, I'm on lunch break anyway.)
I agree with you in general. I don't think something Hellstorm or Pchan or anyone says is wrong *just* because it comes from that person. However, past history *does* lead me to take even things they claim as factual, with significantly more salt than from other people (after all, if they goof on one fact and don't correct themselves, what assurance do I have that the rest of their facts are any more solid?). And it also leads me to examine their logic more closely to see if it actually makes sense.
And no, I don't think most people *do* admit when they're wrong (although if it's something really blatant and really obvious, it's pretty damn dumb *not* to admit your error). But that's not so much because they *shouldn't* but because most people arguing on the internet are not, shocker of shockers, very mature and reasonable arguers. So it's not so much that I expect people to admit their errors, as it is that I am pleasantly surprised and very impressed if they do.
I agree that you shouldn't write something off just because of who says it, but I don't think you should simply ignore their history either and give them the benefit of the doubt as a potentially capable arguer, if they've demonstrated repeatedly in the past that they are *not*. (There's a limit to how far benefit of the doubt will take someone.)
When I'm arguing, my goal is not to be *nice*. My goal is not to *care* about people and *value* their opinion and stroke their ego and make them feel like what they're saying is oh so very *meaningful*. I'll leave that to their mothers. My goal is simply a competition of arguments: factual basis + logical reasoning => meaningful conclusion.
If their facts include things I had not known, then I incorporate that into my view and possibly change my conclusion. If their reasoning makes more straightforward sense than my reasoning, then I acknowledge that and again, possibly change my conclusion. And I expect others to do the same upon hearing my argument, so that once all the facts are known to both sides, and once all the reasoning is known to both sides, then both conclusions will be the same. Or at least both sides will acknowledge that the other has a valid conclusion that they happen to disagree with as a matter of opinion. (Depends, really, on whether the conclusion is itself factual or opinionated.)
If someone is illogical, I will say so, to shoot them down. If someone has their facts wrong, I will say so, to shoot them down. It will accomplish nothing to *weaken* my argument by incorporating theirs to try to reach a shared conclusion, if theirs is based on inaccurate "facts" or logical fallacies rendering their conclusion meaningless.
They can be the sweetest nicest person in the world whose feelings nobody would want to hurt (although it's certainly more fun to shoot them down, if they're an unlikeable asshole), it doesn't matter. If they get their feelings hurt because of their own fault, well ... it's their own fault. I may be sympathetic to their intentions, but that doesn't mean I'm gonna pretend their argument is a good one if it's not.
The only thing reputation of past arguments affects, really, is an indicator of how likely their argument is to be worthwhile before I read it. And as an indicator of whether I should maybe double check their facts or closely scrutinize their logic. (This is why I actually respond to Hellstorm's points - much though they usually hurt my brain - rather than simply saying "Hellstorm, your argument sucks" even though that often ends up being my conclusion.)