Because the final lineup stands a good chance of being Vaughn in RF, Eloy in LF, and Sheets or something like that at DH, with Robert in CF between them, and to get there Vaughn will need RF reps in games. In his first game out there last week he badly misplayed a ball, which is better off happening now than in the playoffs. Eloy in LF would be fine in this setup, except he is still recovering from his most recent injury so they are DHing him.