I was listening to MLB radio on XM today and they were discussing the Arozarena trade. Eduardo Perez said that he talked to the Rays GM shortly after the trade and asked why they would trade Liberatore. The response was that the Rays thought so much of Arozarena, they wanted him. They felt he would be a star in the MLB.
I think that pretty much sums up why the Cardinals traded Arozarena instead of another young OFer in the trade. There was almost no one who complained about the trade at the time it was made.