Now my main question is: how likely/possible is it that the grounding circuit fails partially. In other words: that some of the spark plugs are alive in a certain grounded circuit, and other are not? If the only failure mode of a magneto is a complete, 100% failure, then of course you can detect whether the grounding circuit works by switching L/Both/R/Both. But if you have a magneto system (or electronic ignition, or whatever) that can fail in such a way that some spark plugs in a circuit are left "live" while others are grounded, then a dead-cut check is the only way to find out.
Where one isn't grounding plus or leads, but instead grounding the magneto, one can't have a "partial grounding." An engine may continue to run when hot particles in the engine keep it running...it does happen that engines continue sometimes to "diesel" or chug on after they've been shut down, but that's another matter not connected to the mag.
If your mag is inadvertantly switched to off, let it die, restart, and perform your check again.
Why the need to determine if the mag is grounding out? Yes, you can do a mag check to determine that the mags are grounding and that the p-leads are intact and doing their job...but the prop should be treated as though it's hot all the time...whether the check is sucessful or otherwise. Much like handling a firearm; the safety isn't the mechanical safety on the gun...it's the shooter's straight finger, and his brain. The safety for the propeller isn't the grounding of the mag. It's the common sense respect accorded the propeller. One should always assume the engine can and will fire if the prop is rotated, and treat it accordingly.
What the mag check really does, during a post flight run-up, is act as part of the post-flight inspection and is really for maintenance purposes. Just like finding a problem on the post-flight walk-around, a post flight run-up gives you a last chance to catch a problem and get it fixed.
You'll find that Continental and Lycoming both give detailed descriptions of what should be done, and how it should be done, for a post flight run-up...which includes the idle mixture check you should be doing each time. As others noted, it's probably not being passed along, and too many pilots simply go through the motions because they've seen others do it.
Not everything you need to know is in your pilot operating handbook or aircraft flight manual. Remember, that manual was put out by the airframe manufacturer...publications are also available from the engine manufacturer which give valuable insight into properly operating that engine.