Putting them into the bullpen is a perfect way to get them experience in the SHOW. Next season they either stay in the bullpen, go back to AAA to start if the Twins have a full rotation or prove they belong in the Twins rotation. Many good teams around the league do that. As Cap'n Piranha pointed out the Twins did the exact same thing Johan Santana, and he turned out to have a pretty solid career. Plus, as you pointed out Rogers is probably a future bullpen arm anyway. Next season the Twins rotation will have: Santana, Gibson, Hughes and Milone for sure. May, Meyer, Berrios, Rogers and Duffy are all options for the 5th rotation spot. You might as well get Rogers and Duffy some experience this season in the majors via the bullpen, because those are 3 good players those two are going to have to jump to be able to make the Twins rotation.