I find it useful as an extra level of proof-reading, I certainly wouldn't recommend replacing a human editor or proof-reader with TTS software. Personally, I use it as a final check. Usually it doesn't find anything, but very occasionally it'll find a word that looks sufficiently similar to the correct word that humans don't notice it, but when it's read out it's very obviously wrong. Unfortunately I can't think of an example right now, but it has found errors that humans missed.
A good editor will find a lot more errors than TTS will find (as you say, TTS won't pick up on misplaced apostrophes, but an editor will), and if you have an editor go through it first, there probably won't be any errors for TTS to find, or very few. Editors are human, though, and I'm paranoid, so I like having an extra layer of testing. To put it into perspective, I think TTS has found one or two mistakes out of four books, so it's contribution is very small, but since there's not much work involved in letting a computer read to me, I think it's worth the effort.