I think it would be a reasonable argument that if you need to fiddle with navigation while driving, your nav system failed. And honestly, whether I am using Waze on the phone, or built-in nav, short of "crap, I've put a wrong destination in" I've never had any particular need to do anything to the nav system once I start going.

Now, sure, I can see how a large touch screen might make a better interface for navigation only, as long as you are stopped and can use it without having to look at the road. But I know of an even better interface -- give me keyboard and mouse, and even with desktop Google Maps I'll get routes faster than with any touchscreen system out there.

But of course 1) mouse/keyboard in a car isn't very feasible, and 2) that touch screen, since it's already there, is being used for controlling other systems in the car, too. And that is a problem. Anything that may need a quick adjustment while you're driving (and in real world, of course, that includes things you shouldn't really be doing while driving...) should be doable with muscle memory and at most a quick glance. Touchscreen doesn't do that, so I think it clearly does fail the common sense test.