Not sure I understand every subtilities in you mail. English is not my native language.
It is difficult to give you any scenario as the Rang III is not out yet, and we don't know how difficult it might be to reach some "hidden" functions.
Imagine just that you as a guitarist want to alter the feedback rate. The RANG II wants you to enter a so-called programming mode (press X, then Y to cycle to the needed value). Wouldn't it be easier to use an external midi controller to adjust the feedback on the fly?
If you have several instruments with different output level, wouldn't it be useful to send a midi CC from an external controller so the RANG can change its input Gain?
Perhaps the majority of players won't need this specific extra control and I agree that for reducing the costs and the size, the number of pedal should be limited.
But I think that not providing a way to expand the control might be a mistake.
Why is everyone complaining about the RC50? Because they want to access some functionality with midi and that part is not working properly. It can be to synchronise a flanger to a loop, to start an external drum machine (archaic or not) or just to sync with other musicians.
I can understand you point about musical instruments.
So much people, so much ways to make music. That's the interest of it.