How much should we draw the distinction between players who play a role because that's the role they fell into, they're doing well, and there's no real need to rock that particular boat, rather than what they might have been capable of if the team had pushed them to their maximum potential? Papelbon's upside as a starter is a possible example, Masterson if he'd stayed in Boston as another.
With a guy like Weiland, or Bowden, or Tazawa, or Doubront, if they mature as relievers instead of starters because the Sox's roster was so crowded, but COULD have settled in as half decent starters, do we count that the same as a player who is in the bullpen (like, say, Delcarmen) because he doesn't have starting ability at all? Or is it really all about what a player PROVES they can do, and unproven potential doesn't even count? How do you parse that out?