And I ask again, WHY would you do it in any aeroplane anyway? What is the reasoning?
Sorry I could not reply to your first request for reasoning, I was outside clearing snow...
The mag switch closes a circuit to "turn a mag off" - it grounds the mag out. The fact that the "L" and "R" positions of the switch will turn off each mag in turn, is not a test that the switch "off" position is working to turn them both off. There are AD's on Mag switches for incorrect function.
I was trained to do live mag checks, at dead idle, as required, so I do them after some flights. This was vividly reinforced for me one day, while hand propping a friend's 150 (which I also flew regularly). I called "Mags off", he called back "mags off". I swung the prop to pull it through, and it started. Happy I always swing props like it's gonna start. I gave him a rather disgusted look through the prop arc. He held up the keys for me to see.
Twice, I have had to key "Off" an engine, as someone was approaching the prop, and I wanted it stopped fast. And, in a floatplane, sometimes you want the engine off at that moment, not idle cut off from now.
So, after careful reasoning, and some experience, I do live mag checks - at dead idle only, and for the briefest moment. Just enough to be sure it stopped. I've never had a problem doing that. Not every flight, if I know the plane, just time to time, or before water docking.
For aircraft types where the mags are switched by only two toggle switches, I would agree that two individual mag checks sum to the whole check, but for key type switches, I'm not convinced....