Obviously...
If one's intentions are to employ & implement a full pendulum stroke & the elbow moves, drops or raises then there is a hiccup in the implementation & execution.
Of course one will get a different result than planned if one does not follow the plan.
But if one's focus & intention is to move the cue straight in all planes then the elbow will move as required for that operation.
Unless there is a hiccup in the implementation and execution, which is more likely to happen with a shoulder drop.
Hitting where one intends on the vertical axis is as important as on the horizontal axis at anything above the very beginning level.
To have the cue tip moving on a series of arcs is not most conducive for hitting precisely on the vertical axis, IMO. Please note that I said 'conducive'.
That's just your opinion, which does not represent the actual facts. As has been pointed out numerous times in this thread already.
To do that, basically requires a 'perfect' set up of the pendulum apparatus.
Why do you try and make that sound like a bad thing? That is true of any type of stroke.
If the apparatus can not be perfectly set up, as is very often the case when actually playing the game vs practicing drills & especially on bigger tables, then an 'adjustment', like dropping the elbow, must come into play or a mishit on the vertical axis is almost certain.
That just is not true at all. If you can use any other type of stroke, you can use the pendulum stroke. Position of balls or table size has absolutely nothing to do with it at all.
There is a thread in the instructors' forum right now where the OP has basically opted to use a pendulum stroke for low & a piston stroke for high.
I am NOT looking for any argument or even any deep discussion.
Then don't post on a public forum.
I'm merely putting out some food for thought for those that are interested & may be considering what direction they might want to take their stroke & their game.
Best Wishes to All.
Then you should make sure your "food for thought" is accurate and has some merit if you truly are considering others learning of the game. Misinformation helps no one.