Let's be realistic here.
The typical family buys 1 portable nav unit, keeps it in the glovebox 350 days a year, and moves it from car to car to car the other 15 days.
A portable nav's biggest benefit is to be there, sitting quietly in the sweaty glove box, until that time that WHAM you're lost or need directions. You reach into the glove box, you mount the thing on a beanbag or a suction cup BOOM problem solved.
An unwieldy iPad can't be the Johnny-on-the-spot problem solver that a $120 Nuvi is. Though there are a million things the iPad can be, there are some that it shouldn't be and an in-car nav is one of them.
BJ