Our UX designers usually have a story in Sprint X that the developers will implement in Sprint X+1 (The UX designers and the devs/testers are on one team). I think this makes sense because if you don't have screen mockups and clear specifications you can't really estimate the work during Sprint Planning.
However in Scrum you are only supposed to have user stories that provide value to the user. In our case the UX design stories don't provide such value (they are more like a backlog grooming activity). Also usually Scrum coaches don't recommend having complete specifications before the start of the Sprint, a recommendation which I find hard to understand.
So do you see any disadvantages in our approach? It seems to work for us, but it goes somewhat against Scrum principles.