I don't know whether anyone has the answer on the dental decay thing. In my mind, there's so many aspects to good health for a baby and good sleep comes right at the top. So as a sleep aid, we did the dreamfeed. It wasn't really an issue for us as DS didn't have any teeth til he was over a year!
I think the idea is that they do get into the habit of eating then so they don't wake up later in the night. At around 7 - 8 months, you phase it out. My DS was fine after it was phased out, he was well on his way with solids and I guess he just didn't need it anymore.
HTH