In pre-Lion, any version of OS X, there was no auto-correction and when we got that in iOS, I had wanted to have that be made available natively to the Macs for years. And with the arrival of Lion, that feature has come. At first, I had not noticed.
It was only when I read Macworld's post on turning it off that I realized it was there.
For instance, I typed in "this is noly a test". In the past, my Macbook would have left it there until I make a correctly. Now, this is what I see "this is only a test".
So, why would anyone want to turn it off? I understand the issue that the feature might insert the wrong word from time to time but I see that more likely to happen on the iPhone more than on the Mac.
I might consider turning this feature off for only one reason: if my spelling gets worse. Seriously, I have already seen my spelling worsen in the last couple of years. In the past, I would attempt to be more careful spelling out a word or at least attempt to sound out words that I don't know or hardly use. These days, I just muddle through it and do a spell check after the fact.
With the auto-spelling correction feature, I won't even have to do that. However, I may have to reread the doc more carefully to make sure the auto-correction inserted the right word.
I know this much. Had this feature been available to me in elementary school, there was no way I'd be considered for a spelling bee contest.