jbflyer,
Not paying attention to the way you construct an argument (in this case, a post on an internet forum), will lead to a significant percentage of those reading it to dismiss your post regardless of the validity of the content. I'm not saying that's fair, but something to bear in mind if you want to appeal to the largest audience. An analogy would be the unreasonable demands of the typical passenger featured on 'Airline' when they've turned up late for a flight compared to the reasoned tones of an MP debating in parliament. Both may be lying through their back teeth, but the MP tends to be more convincing.
Bad grammar, spelling, or excessive use of TXT SPK (perhaps that's a generational thing), dilute the potential impact of your message. Now you don't have to turn into Wordsworth before clicking the reply button; but a little care and attention (re-reading your posts, using a spell-checker, etc), will not only ensure your point is clear, it'll convey to your audience that you have thought carefully about your words. And in an internet forum, words are all you have.
I'm not trying to patronise, by the way. I think everyone has the right to put across their argument, and everyone should be given a fair hearing. My advice is simply to help you see the importance of clarity. I, personally, failed English at school; but spell checkers are my friend.