If you pg plays at sf in the blank line up, that means he'll best serve the team in that position according to the game engine.
He could eventually play better at pg, but there is no available sf to play better than he would do at sf. On the other hand, there are plenty of pgs that can come close to what he would do at his natural position.
Anyway, I just tested the blank line ups to see what the game engine thinks about this team. Game engine choses what would be best for the team without taking the opponent in consideration. This is why I'll not use this tactic in a game that matters.
Makes sense?