I haven't Word, but MS Works. It has a similar autocorrect function and when I first found this out, I hated it and immediately turned it off. I know how to spell, thank you! And when I don't, THAT'S what spellcheck or a dictionary is for. Proofreading is for the typos. I really don't want some computer program trying to do my proofing (and probably doing it wrong) for me.
For the record, I mostly just type in Wordpad, which doesn't have any such functions. Sometimes wish it had spellcheck, but I can deal. Like I said, that's what dictionaries are for. Often when looking up a word in the dictionary I'll find that the meaning isn't quite what I intended anyway, and spellcheck can't tell you that.
Please note these are just MY opinions on why *I* don't use the thing. If others want to use it, that's fine by me. I'm not judging them as people.