What's important is how the cue is moving during contact. Agreed. This distance is very short. Agreed.
With a proper setup and delivery of a pendulum stroke, the cue, for all practical purposes will be moving in a straight line a little before, during, and after contact. How so? A pendulum bottoms out only at one point for one instant. Any vertical movement during this time will be extremely small and will not effect the shot. This is where I take exception. ANY difference will garner a different result. To say differences are insignicant depends on whose point of view & why. Additionally, the mechanics of a pendulum stroke provide great consistency and accuracy, and it's easy to learn and master. Again how can it be more consistent & more accurate than a cue that is moving in a straight line? IF, the pendulum set up is 'perfect' the tip might be traveling near 'level' through contact but not if the set up is off. If the set up is off the tip will definitely be arching up or down. IF is a hugh word for being just two(2) letters long.
To accomplish the same things with a piston stroke, it will take an extremely long time to learn and master. Again, I disagree, IMHO the pistion stroke is more natural & simple to accomplish if one just focuses on moving the cue straight & allowing one's amazing mind & body to do what it is more than capable of doing. Until a piston stroke is mastered, there is no guarantee the cue is moving as straight as a pendulum stroke during contact. Again, if the pendulum cue is only moving straight for an instant during a pendulum stroke & continually in a piston stroke, how can what you say be accurate? And...what guarantee is there with a pendulum stroke? It requires a 'perfect' set up & even then the window is extremely small. Perhaps a piston J stroke finish would increase that likeihood. I will acknowledge that a pendulum stroke is probably easier to teach becasue a straight line stroke is more natual & does not really need to be taught. The instruction with a straight cue stroke is more about clearing any road blocks that might inhibit it.