They are fungible, but not in the same way they used to be. Instead of it being that any old failed starter or two-pitch prospect can turn into a decent reliever, it's that there's a constant supply of hard young throwers, some of whom will stick, some of whom will flop, some of whom will have TJ surgery. Hopefuly when the MLBPA pushes for higher salaries earlier in players careers, young relievers like Kimbrel (back when he was young), Giles and Dominguez will be more appropriately rewarded, as they may never get to cash in (even Kimbrel hasn't hit free agency yet, though he got a nice deal, and would have done fine in arbitration too).
