If the metal frame of a building is bonded to the metal water pipe as allowed in 250.52(A)(2) method #2, then the metal frame is allowed to be the supplemental electrode as allowed by 250.53(D)(2)
With this scenario though, when the metal water pipe is replaced with plastic down the road, the electrode system is lost altogether.
We need to supplement this scenario with a additional electrode such as ground rods because the metal frame of the building will no longer be effectively grounded when the metal water pipe is replaced with non-metalic pipe.
250.52(A)(1) requires the metal underground water pipe to be in contact with the earth for 10 feet or more. As soon as you replace this pipe with non-metallic pipe, you have changed the grounding electrode system, and the GES must then be re-evaluated to meet current code. You don't get a free ride by going in through the back door. This is why ALL work must have a permit applied for, so the inspector may determine if code compliance is achieved.
250.53(D)(2) Supplemental electrode required. Generally we look for 25 ohms or less of earth ground with one rod and water main, so if this is not achived then we install another rod etc..... So metal water main or not 25ohms or better should be the rule.