If you extrapolate from the premise, you can definitely make this work. But it takes way more effort than just learning to shoot the ball in "normally."
That is, for every shaft, there is an angle and speed that you can aim the rail shot full in the face with high inside english and it pocket the ball. As noted above, if you vary the speed, the shot angle, or the amount of english, the deflection changes and the shot will not go in.
But, you can use this known variance as the reference point for another shot. Say, one with less angle, or one with more. Also, there are many combinations of shot angle, spin and speed will have the same ultimate result. For example, a lot more english with less speed may have the same net result, although less speed will then suffer from swerve, not just deflection.
All that said, it IS extremely useful for one particular shot for every shaft. Just figure out that shot and you've got a "can't miss" in your bag....
-td