The importance of grammar is clarity. No one can type every keystroke perfectly, or catch every error. But striving to use coherent grammar and intelligible spelling is about respecting other people who you are trying to communicate with: being considerate.
I'm not a grammar nazi, and this principle actually can allow for leetspeek.... as long as your audience is composed of fellow leetspeakers who will have no trouble understanding you.
But using some semblance of proper grammar is a sign of consideration for others. I'm not trying to agree here with lecturing people on their grammar, especially not here. But for the subject in general, I think that's something important to keep in mind.