I think many people couldn't care less about accurate spelling, grammar, etc. and wouldn't bother to read back their own text before sending it. Personally, I like to get these things right, and will take the time to read through everything before I hit the button, but I think it's important not to get wound up by people who either can't or won't make any effort. Life's too short!
Going back to Apple autocorrect, I'm pretty sure that when I have it turned on, I have to make far more corrections to Apple's incorrect 'corrections' than I do to my own mistakes.
