If you place your shadows on Zone IV, even Zone 0 will fully separate.

And if that isn't good enough reason, it will help those cases where you saved the testing for last, played with some seriously out-of-date film, shot a questionably-lit subject and/or ran a few more square inches through your developer than you should.

Why didn't I think of that earlier when I sneered at Bruce's advice and placed my shadows on Zone II.