I remember reading a thing a while back that said babies learn to speak and articulate themselves sooner and better when you don't babytalk with them, so I would assume the same could be said for older learners. It's more about speaking slowly and in simple sentences, and enunciating clearly.
BUT, I'll admit that I've babytalked my students before just to get certain ideas to them more quickly simply because they tend to either shut me out or to pretend that they can't understand anything that I'm saying unless I use as few words as possible. While I try not to do it and usually succeed, sometimes it's the only way I can convey what I want to a particular student within the time frame that I have.