preferred to come off the bench because it gave him a chance to see what the other team was doing before he got in. I think some guys just do better coming off the bench, and being a spark for the team. He and Saunders seem to really compliment each other coming in together.
I wouldn't be upset with them adding JR to the starting lineup, but I don't see a reason to mess with a successful rotation. He's already getting "starter" minutes, so I don't think adding him to the starting lineup would really change much.