Ill dig up some articles when I have a chance. The difference is subtle but 2-3 added miles to a fast ball due to increased delt strength, at higher levels of baseball, is significant.
Also, in terms of punching power - a 150 pound boxer might be born with 'snap' however power is mass multiplied by speed in physics terms. Increasing the strength of the muscles will increase the mass delivered and potentially the speed as well. So while it might be true that a 150 pound boxer can punch with the same speed as a heavyweight, he will still generate less power because he is lacking the mass part of the equation.
It is true that most of the power comes from the core, however because rear delts are often neglected in training, they are a hidden attribute that though small - can add something to the whole.