We want to see minor league umpires working on the field rather than just warm bodies. Minor league umpires are the people that are trained to do the job and protect the integrity of the game. They're trained professionals.
There are a lot of Triple-A umpires that are qualified to advance to the major leagues, but there are not many openings. Those people need to be compensated. They're going to be excellent big league umpires, but you've got to give them something to keep them in the game.