Most of the people who are considered to be illiterate can perfectly read good grammar; some just don't know how and/or choose not to use it out of laziness. Some people could also come from games such as WolfQuest or others that limit one's typing ability. Heck, I've even seen people make fun of illiteracy and purposely type poorly. I haven't come across too bad of people out there in the public maps lately, but that's probably because I prefer to stick to myself at times. I do know, however, that they're not stupid. Confused, perhaps.
Since I have a pretty high tolerance for most things, grammar doesn't really phase me unless it considers RP. I don't mind chatting. The people I've met who have "incomprehensible writing" are extremely nice and most likely nicer than some of the literate snobs I've seen on Feral-Heart.
The best you can do is to help the folks who would be willing to learn, if you have the patience. The couple I have taught greatly benefited from Muddy's School of Excellence. From the diminishing amounts of literate players on Feral-Heart (at least it seems like it), recruiting some students would increase the numbers. This is by no means a well-thought solution; however, I'd be willing to try it.
Trust me, I was at the bottom for a long time too, but setting the example sometimes really does help. I know when I first joined Feral-Heart, the literate people were my role-models and looked up to them.