The thinking is that you are establishing a grounding system bonded at the service (or 2d building disconnect)and any ground shift would make the ground reference rise and fall with that shift. The reality is dependant on the quality of your ground electrode. That is the reason why I like the Ufer, that also catches the steel in the floor (and walls in Florida). Add the bonding to plumbing and equipment.
If you do get a ground shift from the utility grounded conductor, it will be reflected in anything you are likely to touch and you are literally the bird on the wire.
"Ground" is really a theoretical condition anyway. We measured up to 35v between buildings in some surveys we did. As soon as you start stringing other metallic paths between buildings you have to really start thinking about bonding everything to a single point. That will be the 4th wire.
In our data applications we did use a separate EGC and we supplimented it with big bonding wires run with the data lines. There ended up being a lot more copper on the ground side than what we had on the phases.
