I believe the NEC prevents the USE of the water pipe AS THE GROUNDING CONDUCTOR if it contains any plastic. You are not prevented from bonding the conductive plumbing pipe to the electrical system ground. If, for example, you have copper water pipes in the house but have a plastic pipe or coupling going to the street or well then you are still required to bond the conductive pipe to the electrical system ground. That prevents the water pipes from every being energized to a different potential than the electrical system ground. The electrical system is required to have a grounding conductor that goes to a ground rod....
I usually follow your leaps of logic, Bob, but this time, it's a leap of illogic that brought this answer. The simple act of bonding the copper piping to the house ground puts that piping in 'use' in the ground system. You simply do not know what that piping connects to or what may be connected to it--intentionally or not.
Perhaps I wasn't clear--I didn't specify the prohibition applies only to completely new construction--but then again most older homes are and were grandfathered in by the simple fact that most of those plumbing systems were completely made of metal, and that those systems
were already used as as a part of the grounding system--that must be maintained as installed.That isn't all, however, since you seem to contradict your own reply by the statement that "...That prevents the water pipes from every being energized to a different potential than the electrical system ground. The electrical system is required to have a grounding conductor that goes to a ground rod." Let me specify--if there was that sort of connection and the ground rod or its connection failed, that may well put a lethal charge on the ungrounded water system!
I think you'll agree that picking at nits can sometimes blow up in your face, can't they? 73!