Grammar is used to structure a text and make clear what the author means. Without grammar it gets really hard to make clear what you mean. For me, as a non-native speaker, proper grammar and spelling is even more important. This is because I don't have the instant associations which native speakers do have, even though I am quite proficient in English (even if I say so myself).

That being said, commenting on someone's grammar or spelling over the internet is usually wasted effort, even with the best of intentions. It is often seen as a personal attack and takes all intelligent discussion out of the thread. As a result, I never comment on grammar or spelling. I do, however, think that most spelling errors can be prevented since we have all those spell checking programs build right into the browser.

I do agree with you, you are not turning in a paper for a grade, but you are trying to communicate an idea. Grammar can greatly improve the clarity of any piece of writing and help you make clear what you mean. Anyway, just my 2 cents ;-)