I think you are missing the point, maybe not. Sups covering some lame no-show is one thing and that person eventually will be fired. I see it all the time. Filling a need in a crunch is one thing but blatently not bringing someone in then seeing a sup do the work instead is another. The "Flexibilty" you speak of would only open the door for more sups to be doing the work and less hourlys. Hence, nothing to stop UPS from hiring 220,000 Sups....
What kind of merit system are you speaking of? You either do your job or you don't. It's not that technical. The packages either get delivered or they don't. That's what OJS's are for. You can't use a merit system because no two routes or areas are them same (i.e. same amounts of stop, packages, weight, miles , location, dock time, walk time, elevator time, etc.) That's why time-studys and WOR stats are not in the contract.
What kind of open positions are you refering to as well? Management, fill out a resume. Feeder, seniority will always prevail because experience and safety. That's a lot of responcinility and money on the highway. Route bidding, You'd have hard time convincing even Big Brown that the fastest worker should have the easiest route. So, two senoirity based openings out of three and then there ain't much else as far as openings go. Unless you are talking part-time. I find it hard to believe that someone who works part-time 4 to 5 years only to wait to go driving is not pulling as much weight as 6 month employee fresh off the street, Otherwise? how did he last so long? If he didn't like the manual labor part-time he won't like it full-time.