after reading the latest explanation, i still cannot help but think there would be some sort of phase issues, considering your example was in the context of using guitar tracks.
most people here quad track, and that involves separate performances to prevent phasey seashell tones--i'm no theoritcal genius by any means, and it's not like i have any hardcore credentials when it comes to the recording game,
but if you're bouncing a wav of two guitar tracks down to one, and then reloading it back into the same project, you're doubling over performances that are exact replicas of themselves, and that's where my mind perceives their to be phase issues. Yes, this other track may be "lower in volume," but i still the phase coherency problems would still exist.
Maybe i'm still missing a ton of the points you're making here, but i feel that regardless this "close effect" you speak of going right down the middle (i.e. panned center) would be better executed by not using the same performances by copy/pasting.