Best article I've read on the bench press for baseball players
by Steve Zawrotny
Here's an interesting quote from it as well
Also, I have also read an article showing pitchers untrained in bench press gaining 4% velocity from 10 week bench program (also did back work I believe) While a group that trained upperbody with medicine ball work did not show a velocity increase.
Now all that being said I think that musclular imbalance can be brought on a player if he does in fact only work his chest. Imbalanced lead to injury and such. But at the same time I have no problem with pitchers doing either the bench press or the pushup, when done correctly within a well rounded training program.