Hi Ducati and All,
I understand your point. It is certainly another aspect of the dilemma in choosing stereo vs. mono.
The way you suggest is how The Grateful Dead did it, but not how most do these days. While the idea of putting all the sounds of an instrument in its place according to the stage seems like a cool idea, and it worked with The (Original) Dead, it certainly pushes the issue of compromising listeners outside of the coverage area. In this case however, it is the entire instrument that would be lower in level as opposed to only reducing the section that is panned the other side.
As far as keyboards are concerned, the output is often stereo. How wide they are panned is the mixers choice.
Happy Recording,
Marc