It's not switching off the mags that's the problem, it's switching them off and then on again!
Quite.
This is the story from the OP:
I've also been advised by several instructors that switching off both together, and back on, checks whether either circuit is live with mags off. Sounds reasonable.
I think I’d tell those instructors that they should consider what they’re advising from a technical standpoint.
If the “and back on” was left out it would sound better. As it is, no. If the engine stops when both mags are off, you’ve proved they’re working properly. If it carries on running, you have a problem.
Anyway, you should always treat an engine/propellor combination as liable to start at any time without warning. That way you won’t be caught out if by some miracle it happens...
And I ask again, WHY would you do it in any aeroplane anyway? What is the reasoning? Surely we don't have pilots doing daft things they haven't thought though?
Beats me. It’s not unique in the collection of odd behaviours with no rational explanation exhibited by pilots who should know better. Maybe it’s because there isn’t terribly much in the way of supervision, guidance and for want of a better word, oversight, once people are qualified and fly in GA. Plenty of space for funny little habits to form, especially if you’re an instructor somewhere out of the limelight.