I think there are a couple of scenarios where NAV can be very useful.

I can read a map and travel hundreds of miles to get within a mile or two of my destination without a NAV unit. It is the last few miles of unfamiliar territory where the NAV would be handy, especially in heavy traffic, so you can be alerted to the streets where you need to turn well before you see the street signs and without have to try to read each street sign while simultaneously negotiating heavy traffic.

An example; I was driving from Los Angeles to Carlsbad, NM. You just get on I-10 in LA and travel East to El Paso, TX. In El Paso you get off of I-10 and take a smaller highway back up into NM. The only map I had of El Paso was a multistate highway map that didn't quite agree with the road names that I was seeing, it took me 10-15 minutes of driving around and false turns to get on the correct road and even more time to be certain that I was on teh right road.

Another scenario; you (or your wife or daughter) are in a strange area (perhaps at night) trying to find the highway when you realize that you have made a wrong turn, are lost and in a bad part of town where you would really rather not have to stop on the side of the road to try to read a map.

To me it all comes down to price. If NAV was a $50 option then it would be a must have. As a $1,000 option then maybe I can do without.