Everyone has an opinion, so here's a differing one. It can be a good system but your statements are way too general. There's much more that goes into being able to say "PBS is great". Someone just breaking into the airline biz needs to understand that. Knowing what I know now, if I had a choice of two identical airlines - one PBS and one line bidding, I'd steer away from PBS every time.
Ask any UAL pilot what their first iteration of PBS was like and I doubt you'll find any who were junior who say it was "great". The top half of the list bid for what they wanted which typically involved a balance of days off and work. This resulted in an abundance of flying left to assign by the time they got to the bottom half of the pilots. So, junior pilots were maxed out every month flying 90 hours of hard time. To make matter worse, since most flying was assigned, there was very little open time so trip trading was close to impossible. I imagine it's gotten better there, but that's completely up to the negotiating power of the pilot group.
The bottom line is anyone working under PBS doesn't have a choice, so the only option is to make it as good as possible. That falls on smart pilots negotiating the rules, a good software program to follow those rules and the ability to continually update and improve when appropriate. If you have those, then as you said, it can be great. However, PBS will be manpower negative by virtue of the efficiency it brings. Less pilots at an airline is rarely a good thing. That efficiency will typically result in less open time and reduce the ability for people to massage their schedules. In the unicorn, ice cream and blowjob world, no one under PBS would need to massage their schedules because everyone gets what they want. The reality is, that everyone gets what their seniority can hold - far more strictly than under a line bidding system. My situation is a perfect example. Under PBS, I get what my 40% seniority can hold - period dot. Under line bidding, I can go that route if I want to put minimal effort into my schedule. However, I also have the option to bid conflicts or a secondary line and use the necessary follow on bidding process used to fill uncovered trips to effectively increase my seniority and access trips I couldn't get under PBS. Senior pilots get what they want under either system, so their opinions on which one they prefer matter far less. Ask the real junior guys what they like. Some know nothing different, so they may not have an answer. The bottom line is, with line bidding, there are more options available for the junior pilots to create a schedule that suits them than simply relying on what their seniority can hold via PBS.
Trip design and typical schedules at FedEx aren't the same as the pax world, so that may be another reason why it wouldn't work very well here. When it comes to vacation, there's no way that a PBS system can generate the same results as the one at FedEx using line bidding. PBS would decimate our vacation system. I can take the 4 weeks of vacation per year I have now, put them in 4 different months and take all 4 of those months off with full pay. Do that with PBS. No pilot at FedEx who truly understands all the implications would ever vote for a PBS system. That's not out of ignorance or not bothering to learn the system. The only way it would come to be there is the same way it came to be at many of the airlines that use it now. It was forced on them during bankruptcy negotiations. If pax guys have been able to take those lemons and make lemonade, good for them. I'm happy PBS is working for them. I effectively use a version of it when I bid for my secondary lines since the process is similar. Take a pot of uncovered trips, put in for specific trips or general "here's what I want and/or when I want to work" and you get what you can hold. Everyone doing that at FedEx would completely suck.