I find the sensors to invaluable, as I find mirror mounted camera is too small to be beneficial for much other than parallel parking in tight spots. For most other applications, I will use my side view mirrors in addition to my parking sensors. However, the large console mounted screen of GPS equipped vehicles may provide a large enough image to where the sensors may not be required, but I have never seen it, so I do not know.
For those that have the nav option, how does the large screen assist in viewing what is along the sides of the vehicle? I have a tight fit when backing out of my garage and find the sensors valuable in judging the clearance. The sensors really compensates for the side view mirror distortion.