Originally Posted by ismi

There's two issues here. One is language acquisition. I don't know what studies are done, but the anecdotal evidence I've seen all says that if the family cues, the child can learn to cue natively. (Still not without speech/language therapy, but it makes things quite a bit easier by extending the child's language exposure outside of speech therapy.)

Issue two is post-acquisition language. My understanding there is that the CS is not critical. That is, it's still useful in the sense that cues make lip reading easier by disambiguating the mouth shapes; but take away the cues, and the cuer at least still has a phonemic background for their language, which makes it easier to learn to lip read. Rather than try to learn to understand language by lip reading from scratch, CS gives a foundation for lip reading to operate on top of.