Austin Martin gets to develop and improve in AAA playing every day. Sitting on the MLB bench and playing once a week is not a good situation for his development. Plus, burning a rookie's cheap service time when they're likely to be replacement value is not a good economic decision. You can always find a bench player for cheap, let the rookie develop in AAA, and then play the rookie when they're ready to contribute in the big leagues as an everyday player.
I don't think this is about Austin Martin anyway because I'm pretty certain he's going to end up in St. Paul regardless. This is about Hicks versus Nick Gordon or Trevor Larnach and which one complements the starters Kepler, Buxton and Wallner best.