Hi techmad, you create a zone of equal potential across all exposed metal in the area, which is fine. If you then run a main earth back to the con. unit the theory is if a serious fault occurs on a different circuit the voltage on the external earth wiring could rise considerably before the protective device operates to cut off the faulty circuit,that voltage could then appear on bathroom earthing. It would need at least 2 faults simultaneously to occur for it to be a problem, but it's poss.
The new 17th edition look at this in different way, like continentals no more bonding in bathrooms/kitchens, but all circuits in area Must be on RCD's