I see your point, Eddie.
It's "safe" to fly along blissfully ignorant of the fact that the engine has a "dead magneto", but "unsafe" if you know about it.
And if you're flying along on one magneto, and turn it off, naturally you won't be able to turn it back on.
Best instead to fly along blissfully ignorant of the fact that one of the magnetos is dead. Knowing that one magneto is dead would of course be irrelevant to any in-flight decisions.
(By God it's lucky that piston engines are simple technology for simple people, and
almost idiot proof. But they're still out there.
For the non-idiots:
The point of an in-flight magneto check is to check the
ignition system (note: not just the magnetos), and check the ignition system
properly.
The 'standard' magneto check on the ground tells you very little about the health of the ignition system. It tells you the ignition system is not currently having a heart attack.
An in-flight magneto check, LOP at altitude and high power, will show up the early symptoms of ignition system problems, so that they can be cured well in advance of a complete heart attack.
But you have to understand how engines run, and have an engine monitor and know what it's telling you, in order to get any benefit from the procedure.)