The reason I am "sensitive" to grammar, even on internet communications.. by which I mean, "it matters, at least somewhat", is that it is one thing to be educated and knowledgeable of the meaning of the words you use, and then choose to take shortcuts, or employ poetic representation, but quite another thing to be oblivious and ignorant of the traditional rules of the language you are using.
It's OK, by me, to be lazy or take grammatical shortcuts... it is not OK, by me, to be ignorant of the usage of the words you choose to use to express yourself. It harms the perceived quality of the information you are attempting to convey.
Who are you more likely to listen to?
Person 1: "Their is raining to day"
Person 2: "It is raining today".
If one is to argue that they are speaking in a forum where the grammar of their sentence structure does not matter at all, then I would argue that the statements that they are attempting to make on the forum are equally thoughtless.