Signing Demps is potentially improving an area of the game that the Pats are not strong in. Using him as a KR and possibly improving that part of the game makes it a good move for Pats.
He will most likely be eased into the offense at RB. I could see him being used later in games when the opposing defenses are gassed.
The Jets already have that area covered with McKnight and Demps isn't a ground and pound type back.
That is why the press would have crucified Rex and they praise the Pats.