Exactly. I learned to fly in the early '70s and they didn't
teach us to look for a hot mag. I learned that when upgrading some
years ago. Doing this, I found that one of our 172's mag switches would
allow the mags to keep firing if the key was forced against the "Off"
stop, and upon investigation found an AD against any of those switches
that did that (76-07-12). Bet there are still a lot of them out there.
As far as the discussion about a single impulse mag rather than
two: Lycoming may have recommended a change to one to get rid of the
impulse coupling on one mag. The springs in these things have been
known to fail, and with two mags one might fail and not be readily
noticed if the pilot is a careless twit who doesn't do a decent runup;
if the other spring then failed as well the timing on both would go to
near zero BTDC. Power output would be low indeed. Corrosion has been a
problem in engines seldom flown or flown on very short flights, and the
springs, being a high-carbon steel, corrode quickly. The Bendix dual
mag (one housing, one gear and impulse coupling, two mags) had an AD
against it to replace that spring with a better one, since its failure
could and did cause accidents.

Dan