The points made by PNH and moondog above on guides are valid, most of the ones in China aren't worth a flip. For my folks' trip, we decided to take a chance anyway, based on the specific situation:
1) On a logistics basis, the local guides were there to deal with language issues, and keep arrangements running smoothly to minimize downtime. My parents knew they would likely not be making any future trips to China so they didn't want to waste time on the ground. They knew the days with guides would be over-planned and a bit frenetic, but they put in a few free/rest days or half-days with no guide every so often, where they could just wander around getting into/out of trouble. Neither of them any longer has any language learning talent or inclination to deal with phrasebooks and they were very intimidated by the language barrier and the thought of having to argue their way through 1+ billion people.
2) My parents are pretty experienced travelers and have lived in Asia before (though not China). They are well acquainted with Asian cultural propensities (not just a Chinese phenomenon) for "faking it" due to ignorance, laziness, not wanting to lose face, and/or need to put the official government propaganda spin on things. Also that you need to discount over half of what you hear while still keeping a straight face. As their trip was more generally focused on long-ago historic and general cultural stuff, they figured they could let the fluff run in one ear out the other. And by using a series of local guides, if they crapped out with one, it was a self-limiting problem, and one they could take care of by moving on or releasing them, without affecting other parts of the trip. And since they only used local guides, the guides didn't need to have knowledge of all things China, just something about their local vicinity. From my folks' post-trip postmortem, they thought the guides they got added enough value to make it worthwhile, knowing full well they were not going to be given complete accuracy or comprehension anywhere.
3) My father is very much a pre-trip researcher and guidebook/history book/ , biography type of person and did a lot of homework before coming for his China visit. Theoretically, they could have done the sightseeing without a local guide except for two things: (a) My father can't retain all the information any more and gets muddled up. And he's not the most organized presenter of information. (b) Most importantly, my mother can't stand to have my father do his monologue while playing "tour guide" for the two of them. This would have made for a very unhappy dynamic within less than one day, whereas she'll put up with a local guide warts and all. I'm sure the guides had no idea they were being used as an intermediary peace-keeping team of one, not just an information and logistics provider.
