Another factor to keep in mind:
The minor league seasons aren't over yet - they wrap up next week. As much as the minor league teams are about providing talent to the majors, you don't want to totally abuse that by calling up a bunch of players who are just going to sit on the major league bench and leave the minor league teams short-handed. Calling up Zimmerman is one thing - he could potentially help the major league team right away, in a relatively significant role. Harris, Short, Godwin, Watson, etc? They're just going to ride the back end of the bench. I think we'll see a bunch more players called up next week, after the minor league seasons end.
Scot.