Where do you get stating unnecessary? LOL And as far as I know most remove the drain plug to change there oil?These plugs are unnecessary. The only way to find out if they picked up anything is to remove them. The vast majority of vehicles don’t have them.
The point about transmissions and differentials is a good one. These will shed off far more ferrous material yet the vast majority of them never get the fluid changed.
I might add that you do realize on differentials the drain plug are not always at the bottom of the third member casing right?
You also miss the BIG point that seeing what is on any of the magnets could help to diagnosis a pre problematic condition before it becomes worse.