I agree with you. If we agree that the upper swing center is the pivot stabilising point, then the pivot stabilising point must not bob or sway.
The idea of a pivot stabilising point (whether it is the head or upper swing center) remaining stationary is a very sound concept. I have only questioned the belief that the pivot stabilising point has to be perfectly centralised between the feet and the idea that a golfer pivots around a vertical pivot axis that is centralised between the feet. I think that if the pivot stabilising point is slightly left-of-center, or slightly right-of-center, that it can still perform its stabilising role efficiently - as long as it does not bob or sway.
There is no bobbing-or-swaying of Tiger Woods' and Stuart Appleby's upper swing center (pivot stabilising point) even though it is not centralised between their feet.
Although I personally prefer to keep my pivot stabilising point more centralised - like Aaron Baddeley - I think that it is a personal swing preference, and not a mechanical/geometrical imperative.
Jeff.
There are ball flight implications . . . . based on the clubhead path's that can result all things being equal.
Threaded Mode
