There are two cases when you can to use NC + VC pairs:
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You are putting up a modal dialog: you want a title bar and possibly a way to perform some navigation inside the modal, use a NC + VC pair instead of simply a VC
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You are using a tabbed design: each of your tabs has navigation and requires a navigation controller
You can perfectly get away with using a single navigation controller, although this will cater to only the simplest applications. Knowing that you can use a NC + VC pair is very handy in many cases.