I heard exactly the same rumor about the copper shortage in WW2 & I believe that is the reason that the 3 wire installation was allowed. As a side note, the Manhattan Project would have used so much copper for transformers that it would have actually hurt the war effort. The treasury ponied up several million ounces of silver which was made into wire for the required transformers.
As Bennie pointed out, the 3 wire installation has had an excellant safety record. However, the jacklegs in this business started to think that neutrals (wahatever they are) and equipment grounds were interchangeable. I think it is best that we have now gone to a 4 wire installation.
Tom
