If the parallel paths for the grounded conductor current are as hazard, then those paths should be prohibited on both sides of the main bonding jumper....not just on our side.
To me, it's not the parallel path itself we're trying to avoid, it's the intentional connection of a current-carrying circuit conductor to conductive equipment housings and enclosures. The change of 3- to 4-wire major-appliance feeds is a great example.
People have said that the change wasn't necessary, with almost zero problems with the 3-wire circuits, but, I have experience to the contrary. A customer was badly shocked when caught between his range with a brokem neutral, and the sink.
We're all familiar with what happens when a normally-zero-volts-to-earth neutral conductor is broken: the EGC system becomes energized. It could be much worse when it happens to one piece of equipment, surrounded by still-grounded metal.
At least, with an intact EGC system, the various normally-grounded equipment and other conductive surfaces, both electrical and plumbing, are still bonded, acting as an equi-potential grid of sorts. Grounded or not, gradients are minimized.
The main disco enclosure, where the neutral is bonded, is the beginning of the premises EGC system. It's not earth that the EGC is solidly bonded to for fault current, it's the service grounded conductor. The goal is to make sure a breaker or fuse opens.
If one circuit's grounded conductor or its EGC opens, the immediate hazard isn't really increased. But, if a combined-purpose conductor opens, the hazard is increased. At least, because of the still-intact EGC system, all equipment is at the same potential.
