most people think linguistically. 9 times out of 10, if a man can't speak clearly, he can't think clearly, especially in a crisis. to speak clearly, one must follow consistent rules. to form one's own internally consistent structure of grammar seems to be a waste of time, and is actually self-defeating when confronted with a social situation.
so if an adult doesn't use proper grammar, that neglect reveals quite a bit about them.
1. never bothered to learn.
2. does not realize that inaccuracy is a bad thing.
3. overt social rebellion.
4. never taught properly, and unaware of that fact.
5. general laziness.
of those, i find that only #4 is excuseable. then again, i often make grammar mistakes that an englishman would scoff at. <===[good example that i intentionally didn't correct after initial error]
so i guess it's a matter of degree.
one way or the other, it is hard to ignore the fact that, in general, people who are careless in their speech are careless in many other things, as well.
[BTW, failure to capitalize on a web forum is an egregious example of #5]