This is probably the most reasonable explanation. Having him play sparingly for a week or so isn't going to kill his development, and it's nice to have a capable left-handed hitter available on the bench while the schedule allows them to go with a 4 man rotation. Also, I think there was some uncertainty surrounding Mauer's thumb (and possibly Morneau's wrist) when Parmalee was called up, so it wasn't a bad idea to have an extra first basemen around just in case those two were going to miss a few games.