PBS IS A CONCESSION!!!
There is no other way to put it. It makes your vacation, training days, etc. worth very little to you. It prorates everything to the company's advantage. Anybody who tells you anything different is being naive. It doesn't make pairings more efficient. It might let you bid a little more specifically if you're senior, but you pay for it a lot more in different ways. If you're a reserve pilot, you'll get 11-13 days off per month depending on your airline. Vacation will give you an extra 3-4 days off per month with PBS.