How the 40-man Roster works:
Whenever a player gets called up, he must be
added to the 40-man roster, if he's not on it already. If an organization's 40-man roster is at capacity, that could keep
an otherwise deserving prospect down on the farm. To add someone to a full 40-man roster, someone would have to be removed,
exposing that player to waivers. If a team doesn't have an obvious candidate for that, they may decide to hold off.
A player's service time in the organization also
can come into play. A player who is 18 when he's signed can spend four seasons in an organization before he has to be protected
on the 40-man roster. Anyone who is 19 or older must be protected after three years. Once past that time of service, a prospect
must be put on the 40-man roster if his organization wants to keep him from being eligible for the Rule 5 draft, held each
year in December.
In other words, if a team isn't in a position where
it has to add a prospect to the 40-man roster (The Padres, for example, don't have to add pitcher Tim Stauffer to the 40-man
roster for another two years, technically), they may decide not to bring him up this September.
Also, when you add a player to the 40-man roster,
the clock starts ticking on his big league service time. That moves the player that much closer to arbitration, free agency,
etc., though one GM said that because it's only 30 days, that's never the make-or-break issue.