i don't know if it could work like the NBA here, because:
1. Through GS management, many team don't have a classical(standard) lineup, and switch there S5 quite often.
2. In the NBA the award mostly got won by a starter, who just sit on the bench till the possibility to sub him in(or maybe the second). For the Mavs we had for example the former 6th men of the year terry(i believed he was second this year), and it is hard to say that Stevenson is the better player then him. I believe for the most people he is the third important player in the mavs roster(Butler is second for me till his injury) ... And in case of Terry i could also imagine, that he is backup so that he could compete for this title.