Some Pokemon, like Raikou, have now evolved past Sleep-Talk and probably won't use it again. The odd one may, to throw in a surprise, old-school element, especially if their opponent has an anti-calm-mind-raikou solution.
Some Pokes, though, will be so glad to talk in their sleep again. Some Pokemon lost their use after ST wasn't available in RS, but that is about to change, methinks.