It seems like there should be code rule that prohibits using a EGC also as a circuit conductor (current carrying)
AFAIK, the NEC _never_ allowed the EGC as an intentional circuit conductor. The closest was to permit the neutral of certain circuits to be used for bonding equipment on those circuits (range and dryer circuits in the past).
But it is an unavoidable aspect of the physics of the situation that there will be _some_ current on the EGC because of leakage and capacitive coupling. No insulation is perfect and even if it were there is still going to be capacitive coupling. Part of the UL standard for switches was a maximum allowed leakage current.
NEC permits listed switches with their maximum allowed leakage current, which means the NEC permits a certain amount of leakage current on the EGC.
The question is how to treat _intentional_ pseudo-leakage. I say 'pseudo' because it is such low level current that it is lower then the maximum allowed leakage, but it isn't really leakage because it is intentional.
The CMP decided nope, we are going to require a grounded conductor be available or easily brought to each switch location, so that intentional current on the EGC isn't needed.
-Jon