"Virtually all the non-mil pilots I've flown with have been able to use MDR for the diversion aspect of the PPL Skill Test - because we teach them how to." - I was thinking more of the people who come back for refresher or LPC/LST flying and have only used wizz wheels for planning and not practised MDR for a few years. IME they've usually forgotten how to plan a diversion whereas if they had practised the techniques for pre-flight planning (even to check wizz wheel output) they would be totally up to speed when needed.
I haven't had anyone want to use a GPS, but I'd expect them to do the normal planning and map/plog prep then use the GPS for en-route guidance, keeping a written log in case it goes belly up.
HFD