I'm only one voice here, but I think an argument is immediately stronger and more appealing when it's spelled correctly. Spelling & editing come from a different part of the brain from the passionate argument, so when writers temper their passion with reason, they are able to proofread, and be polite. What do you think Rico? Does someone's bad spelling make you proceed with caution?
For people who are speaking in their first language, I agree with you. If someone can't take the time to check that the language they've been using their whole life is correct, I find myself wondering how thorough/thoughtful they've been in coming to their views and conclusions. Of course, this excludes the occasion honest mistake.
Spelling is not always indicative of intelligence or competence, but in the vacuum of the internet where users are already grasping for contextual cues, spelling and grammar can go a long way toward creating the sense that a given user is a-- well, not a "reliable narrator" but a "reliable commenter," as it were. Also, to put forth an argument is to open the door to criticism. The accusation of an ad hominem attack, when the very accusation is incorrectly spelled or applied, certainly gives a reader an opportunity to doubt the veracity of the assertion. So, in isolated interactions, then yes, sometimes poor spelling does make me proceed with caution. However, if I have had prior interactions with a user, then I am more likely to let spelling, grammar or usage slide if I have already seen that they are a "reliable commenter" much in the same way that I would forgive malapropisms or other mistakes with an acquaintance who I know to be intelligent. If you are asking if I proceed with caution when I respond to comments with poor spelling as I think the user might be in an impassioned state, well then I'd have to say no, but I might word my response in such a way that I felt my meaning would immediately be apparent. The old truism bandied about amongst EFL teachers is that "one is truly fluent in a language when one can win an argument in it."