In an online argument/discussion, if you have to stoop to criticizing your opponent’s spelling or grammar, you’ve already lost.
The simple fact of the matter is, if you can understand what someone is saying, you don’t need to correct them. Just because someone misspells a word or two, that doesn’t make their point any less valid.
If you do try to correct them, what you are actually doing is insulting them and their intelligence. Small people do this while arguing in an attempt to make themselves appear bigger and smarter than they really are.
Web sites such as Digg are plagued with people who do this back and forth, and I always get annoyed by it.