David Boyd probably knows this issue better than anyone, so I won't add much except to say that there have been cases. For example, the Town of Hudson case took the precautionary principle, which is one of the principles referenced here and incorporated it as a customary principle into Canadian law. it was the first domestic recognition, and I think the second anywhere in the world, of that international environmental principle.
Over time, as David said, these principles get adopted in enough places around the world so that they do get incorporated into customary law. But none of that would be anywhere near as effective as this. Its only purpose is as an interpretive aid. When a court was interpreting a federal statute, they would use that as an aid to interpreting what was there. It wouldn't of course, as you know, create the right if it didn't exist. Having this bill is vastly more effective than waiting for courts to interpret provisions of bills that only touch on some of this.