It's all because of the "No Child Left Behind" crap.
Seriously, people are becoming too flexible with how kids are taught.
When I do emails or post in the forums, I type with the best grammar and spelling as I can (with a few exceptions like "cuz" instead of "because"). Other than that, I use slang because I'm speedtyping... on AIM, etc.
When my parents use their cell phones to send me text messages, it's difficult for them to do it with proper spelling or grammar. So, they use abbreviations. 1 out of 50 times, I'll have to think for a moment to figure out what they're trying to say... other than that, they do just fine.