Neil and Katya are currently the only dancers on the show who have world titles under their belts. They are the best dancers and choregraphers by miles, and have actually been involved with Strictly for years, firstly as choreographers and then backing dancers before joining the pro rosta officially.
Dancing with the Stars in the US have been 'benching' pro's for years. In fact, its not unusual for a pro to take a season off altogether and go off to do other stuff before returning there and no one bats an eyelid. Gleb even did a series of Strictly and DWTS Russia before going back.