From my reading, the reason it was unacceptable was that since the wires were previously identical, the manufacturers attached an arbitrary wire to the case, so

From my reading, the reason it was unacceptable was that since the wires were previously identical, the manufacturers attached an arbitrary wire to the case, so 50% of the time it wasn't the neutral one. So, once we switched the polarized plugs, some of the appliances would always have a hot chassis; unless we made sure appliance-makers always attached neutral to the chassis.