My understanding on the velocity issue is this. The band exercises work the rotator cuff muscles. Those muscles largely just stabilize the shoulder joint. They may figure into the acceleration of the arm a tiny bit. And they may figure into the deceleration of the arm as well. But they certainy are not the big contributor to velocity.
Now, strengthening the muscles that stabilize the joint will allow a pitcher to safely put more force on the joint. This allows the pitcher to do the other things necessary to increase velocity more significantly. So, stengthening the cuff is more of an "enabler" than a "direct contributor" to increasing velocity.
Someone correct me if I'm wrong