Never had their teeth cleaned. That said, my dogs ate a regular diet of dried, crunchy dog food, supplemented of course with various treats and things along the way. But they always had healthy teeth and never needed cleaning.
I mean if problems develop, sure you'd want to do something, but given good healthy teeth and gums, I think it's a racket. Dogs, like people, should be put anesthetized anymore often than absolutely necessary.
I vote it's pretty much a racket.
btw: My vet degree comes from watching Ol' Yellar and Black Beauty.