Against my better judgement, I'm going to weigh in on this. I think a compass calibration under flight conditions is a worthwhile thing.
I have personally experienced sophisticated navigation and autopilot systems going haywire - admittedly on a yacht, at night. The results are a raft of confusing and contradictory indications and my only means of determining which parts of the system were lying to me was my Sestrel magnetic compass.
I understand that a similar failure mode caused the loss of an Airbus in the Atlantic. Clock compass and ASI - first principles in an emergency.
To put that another way; a man with one watch knows what time it is. A man with Two is never sure.