In my opinion, and what was proven in past performances, he should stick to the closer role. You don't want Papelbon in your rotation. He just doesn't have the stuff to get him through the lineup multiple times. All his pitches would be a bit slower, and not a single guy would be confused by what he brings. He was hit HARD in spring training when he was pitching as a starter, and that's because he just simply doesn't have the stuff to be a starter.
Look at Mariano Rivera for example. He came up as a starter, and got rocked around, and went into the closers role and bam. LIGHTS OUT. And thats because a closer can get away with just 2 really good pitches, rather than needing multiple pitches to change eye levels, and timing. But over Rivera's career as a closer, you know whats coming and still can't hit it.