Reading the posts here I believe there may be confusion between two different,
but related issues.

Why do we have two mags per engine and two spark plugs per cylinder?
The main reason is redundancy, and the secondary reason is performance.

On the performance side, losing one mag in flight should produce a slight
decrease in performance, but no really significant roughness or danger to the
engine. The function of both mags is detected through the single-mag check on
runup.

A far more likely occurrence however is the failure of a sparkplug in one
cylinder. This goes almost undetected if both mags are working, but will
produce very significant roughness on the single-mag check, and will produce a
considerable imbalance in operation. So on the redundancy side the mag check
serves not only to detect a faulty mag, but more likely to detect a faulty
spark plug. If you never did the single mag check, you could theoretically run
for some time with a defective plug or even more than one. Then the day you
have a mag failure you are at risk of a rapid engine failure.