Nicely put Beagle.
Throughout the PPL course a student is learning a multitude of new techniques, many of which will be very foreign to him/her. Map reading has always been a difficult skill to master (except for a lucky few) and, I hope, many of us should remember the hard mental workload of determining how to get back on track once a deviation has been recognised.
The real issue here is developing a student's faith in a technique - and you can only do this if you show the student that you can use the same problem solving methods as you have taught him. Do that and his/her faith in the technique is boosted tenfold - a real step forward to those that find this bit of the course hard.
Used sensibly a GPS can enhance map reading skills a lot. Used badly - the student being fully aware that the instructor is using GPS for positional information - and it degrades the whole meaning of the map reading exercise. All the student will be thinking is that he must get a GPS as soon as possible - it's what the instructor uses!
Beagle's approach is sounder. The GPS is present but the instructor is still showing the pupil that his assessment of navigational accuracy is based on the same technique the student is using. This enhances respect for the instructor and faith in the technique being taught.