The fact of the matter is, discussing topics on an internet forum requires people to be able to understand what you're saying, and if you don't type in the accepted format, confusion ensues. They created a set of rules to follow when writing, so that everyone can easily decipher what you are trying to say. If you refuse to use those rules, then no one will be able to understand the intent behind your words.
On a side note, people typically only ball bust grammar mistakes on an otherwise stupid topic or comment. If you make interesting and/or relevant threads and comments, people are generally less likely to jump on you for your mistakes.
Edit: after my reading my long rant, i realize i missed my point completely. I was going to say that i don't think most people intend to look for, and bitch about, grammar mistakes, they are merely trying to read someone's words and can't help but notice when they do it incorrectly.