There's nothing about PBS that's intrinsically more efficient, it's just a tool to create a monthly schedule from individual trips rather than having to choose from company-created months. The programming that PBS follows is where efficiency may derive. For instance, PBS can be programmed to allow conflict bidding. Realistically no airline would allow for that but the point is PBS is an easier way for pilots to exercise their seniority to get the best possible schedule before having to deal with trip trading. That's the reason so many pilots prefer it to line-bidding.
I know we have our differances but I think everyone would have to agree that if management really wants something it may not be good for pilots. If PBS isn't more efficient (needing fewer pilots) than why does management want it so badly?