I saw the callup as strictly 40-man roster management. They temporarily needed an arm, they didn't want to drop someone in order to call someone else up, so up he came.
His brief tryout in the majors probably helped seal the decision that he wasn't making enough progress. It's too bad to have to make that decision on a 23-year old - MLB rules on protecting prospects probably do more harm than good for some players, rushing them needlessly, and a decision to sign a kid from the Domincan at age 17 involves too much gamesmanship for my taste.
On the flip side, maybe some of these guys get look sees that help them in the long run. Polanco's first call up, while seemingly uneventful, may have been a confidence booster, or given him exposure to decision makers who saw something they liked.
Of course, the flip side of THAT, is the fact Polanco himself was out of options this summer at a time he really could have used one. He had a lousy enough summer, joining the waiver wire Merry go round would've truly sucked for him--and Twins fans.