Well, yes, if you are going to fly in conditions that might actually require a change of route or even a change of destination, you want to be flying with a decent moving map GPS so you can see where you are at all times.
The quick back-of-a-fag-packet diversion calculation done in the PPL doesn't really work when doing it for real under pressure, especially if trying to avoid CAS.
Dead reckoning is OK if everything goes to plan but if you have to divert from halfway between two waypoints and do some random weather avoidance, the plan falls apart fast.