I guess the crux of this is
a) You don't have to buy it. The car is perfectly functional without the nav. Just use a TomTom if you prefer it.
You are right on that point. The built in Sat nav does not need to be used, and I can carry on using my Tomtom, which I will do. The only issue I have is that they have that pop-up screen in the middle, getting in the way of positioning the Tomtom. I would have personally prefered to have a screen elsewhere (maybe lower down in the dashboard), or for Audi to come up with something useful on that screen, so I don't need to keep yet another external device.