There is actually a fair bit of research out there on the functional / sport specific value of medicine ball training - especialy with respect to rotational srength. For example:
Effect of twelve weeks of medicine ball training on high school baseball players. - These data indicate that performing a 12-week medicine ball training program in addition to a stepwise periodized resistance training program with bat swings provided greater sport-specific training improvements in torso rotational and sequential hip-torso-arm rotational strength for high school baseball players.
Comparison of trunk kinematics in trunk training exercises and throwing. - Incorporating trunk training exercises ( the study included medicine ball throws) that demonstrate sufficient trunk ranges of motion and velocities into a strength and conditioning program may help to increase ball velocity and/or decrease the risk injury.
Can't resist this one, just for the comedy of it: