Please note that this is not in any way meant to cause offense or to target anyone in particular - I haven't seen any of the posts this refers to, so if it is an ongoing battle, then I am not weighing in on anyone's side - this is just from my perspective.
While I rarely have the urge to correct people online (I get paid to do it for a living, so I don't give my services for free!!!

) and I agree that grammar/spelling police can be very annoying, still, some people should be ashamed of their poor command of their own language.
To be clear, I am not referring to non-native speakers or people who have genuine problems such as dyslexia, but to people who put no effort into articulating their ideas.
There are two important elements, in my opinion.
First, an internet forum is a purely text based board for communicating with other people from across the world. The grammar and the words you use directly impact upon the strength and clarity of your message. If you don't write succinctly, then you can't expect people to understand you. If your message is convoluted and the grammar is incorrect, then it is most definitely a failure on your part to convey what you mean, no different to mispronouncing a word and expecting the listener to do the work to fill in for you. Given the difficulty many people have communicating face to face, writing anonymously requires a little extra effort to ensure understanding.
Secondly, it is shameful that people do not attempt to improve upon such a critical skill. Writing is one of our species great achievements and literacy is something all societies should strive to maintain in their populace. Why do people take affront at being corrected in something they *really should already know* and *should have learned as children*? Instead, those people need to appreciate that they are being given insight into a critical flaw and one which will seriously hamper their ability to achieve in life. Perhaps instead they could be grateful (as they would be if they were using a second language and someone helped them by correcting their errors) and use it as a learning opportunity.
Pick up group looking for a rouge!!! Did you really want to put some make-up on prior to fighting those monsters?

The key problems, as I see them, are not typos but persistent spelling errors that show that the word is not truly known or that the grammar is not within their command. For example, using "loose" in place of "lose", using "would of" instead of "would've".... both of these examples are completely incorrect use of the language but it is expected that the reader should go to the effort of deciphering this, rather than the writer actually using it correctly in the first place.
The worst I have ever heard, sadly from my own brother, was on MSN. I asked my younger brother (early 20's) how he was and he responded.....
"Im not so good as i didnt slepted last night"
He is my brother, so, naturally, I tend to spend the time explaining the problem. Sometimes he listens, but generally his idea is that "everyone knows what he means" which, of course, is true.... bu ten i cud rit li ths nd ud undastan wha i men.... if you think that's acceptable then we could just go the whole hog and resort to grunting at each other and sniffing each others rears to communicate!

While I agree that the grammar police should lay off or at least make an effort of doing it constructively, the language abusers really shouldn't be attempting to take the moral high ground.