Because trips affected by training and vacation are dropped into the open time pool, which enables the ability to trade not just trips, but days, which doesn’t exist in my BES. Because of that, PBS minimizes staffing, which is why the airlines want it. I spent close to a decade bidding “paper lines” and don’t pine for them because: A) They’re not coming back and B) There are, as you note, some advantages to PBS, but there were plenty of good things about bidding for preconstructed lines.