Well, you asked this:
Quote:
With a Lycoming or Continental engine at idle, briefly switching off both mags will do no harm, and is good practice in my experience.
Why is it good practice? It's potentially damaging and there's no reason to do it, so why would you?
(Your question in bold) I wanted to point out why there
is a reason to do it on some types.
It's misleading and potentially dangerous to put forward as "fact" something which is an opinion based on personal preferences. I've been caught out a couple of times in the past by pilot hearsay so now I always read up the manufacturer's requirements and advise others to do the same.
I've looked up the Rotax 912, which was one of the engines mentioned by the OP in his initial question.
Does it actually have "traditional" self contained and self generating magnetos? Looking at the parts diagram, although there is a magneto excitation coil, there is an external electronic module and another coil shown, too. "Switching off the ignition" may not actually be earthing the system on this type as per a self contained mag, but rather disconnecting the live side. Which means we might be discussing completely separate issues as if they were one and the same.