Magnetic plugs are far more effective in differentials
of all sorts, transmissions etc., where micro metal
shavings are rampant and greatly accelerate wear.
Of course manufacturers install magnets or magnetic
plugs there to begin with.

In an engine they are much less effective as by-products
of combustion not microscopic metal debris is responsible
for most wear.

I am a great fan of keeping the engine oil as clean as
possible.  So frequent oil changes, synthetics, oversized
filters, magnetic oil plugs, upper cylinder lubricants in
the fuel (such as MMO etc.) are all in my book.

It is true that one will probably never wear out a
modern engine in the life of the car, but what
a diference it makes, and great satisfaction (not to
mention saving) when your 56k mile, 8 year old
engine runs absolutely no different than a brand
new 25 mile engine in a test drive car at the dealer's.

This is why the effort is worth it to me.  It is also
why I like well balanced engines like boxers'.
They can run forever, and with a bit of extra care
they can run like-new for a very very long time.

M.J.