With one rig, MY_RIG belonged in Operating conditions and nowhere else.
With several, that becomes the wrong place: operating conditions describe
a PLAN — 'on 20 m I use the beam and the 7300' — while the question a
logged QSO asks is which radio was keying.
So each saved radio carries its own MY_RIG, consulted BEFORE the per-band
station. When the two disagree — a second rig borrowed for one evening on
20 m — the one that is transmitting is the true answer.
Left empty it changes nothing at all: the old chain (band default, then
the profile's rig) answers exactly as it did, which is what every
single-radio station will keep doing without touching a setting.