I must say, I am very much torn on this subject.
First off, let me say that, like Nerd4Hire, I think it's really important to know the correct rules of spelling and grammar, but whether you should at all times abide by them is a whole different question.
Being a non-native student of English at University, I should be all in favour of correct spelling and grammar. I use it on the boards (that is, what I believe to be correct), but not in chat. In fact, when chatting to my fellow-student (we speak this weird English/Dutch hybrid language) neither they nor I speak correct English. We abbreviate everything, use slang words, and never use apostrophes; even though, we should all be able to write perfect English. (We are not sure yet, though; we still have to get our grades for Grammar, Writing, and Translation back! 😉)
I agree that for non-natives it is easier to understand correct English. However, is it not also true that, in the end, a language belongs to its natives and not to the non-natives? Would you accept a collect-call from a total stranger? I think not; you would not want to pay, just because some stranger wants to talk to you. The same goes with language, why would you have to put in an effort when we wish to communicate in your language. It is, after all, our choice to communicate in English and not in our native tongue. The abbreviated language, the kewl-talk, and slang are a part of that language and an integral part of that language when communicating on-line.