Figure this, the Giants or any other Major League Baseball organization (I only used the Giants first because this is Mccoveychronicles) keep five or six mainstays in their bullpen, guys that are solid and consistently get hitters out, whether it is being the closer or just being the lefty specialist, as long as they do their job and hold leads or keep the game close. In other words, perform well. The other one or two spots in the bullpen would be reserved for calling up minor league guys. This would eliminate a hitter's history with a pitcher and the pitchers from triple, double, or even single A would be able to help the team without hindering their progress.  Then when teams begin to develop a book on quick callup minor league pitchers, they can simply switch them with another minor leaguer. Leaving one spot open in your bullpen would allow for this system to work. Another plus would be that teams might be able to find a minor leaguer who is never figured out by opposing teams, and he can become a mainstay in the bullpen.