Why is this important? Who cares if you use correct grammar?
We communicate in language. More and more often, especially with the advent of the Internet, that means written language. Forums like Weblogs, the World Wide Web, and mailing lists are primarily written media. In these forums, we see nothing of what people are save for what they write. In any written medium, people who write clearly and distinctly, and who use language precisely and in a way that is easy to understand, will likely be read more often and given more attention than people who do not.
Anything you do that makes your messages harder to read or harder to understand will make it more likely that people will not pay any attention to anything you have to say. The written word is the only thing you have here; if you do not use it well, then your ideas, no matter how good they may be, will be disregarded.