[RichLockyer]

The drain plug will catch the crap first.

Honestly, I wonder how much of what is on the drain plug wouldn't be caught by the filter element anyways. Overall, the stock system is pretty good. You've got the screen to catch any particles large enough to damage the oil pump, then the oil goes through the pump, then through the filter, and on into the engine.
Wear metals will return to the sump where they would be attracted to the magnet in the plug before returning to the filter.

The difference is, not ALL of the oil in the sump is close enough to the plug for the magnet to draw in the iron shavings. 100% of the oil going into the engine passes through the filter (and half of it would pass the FilterMag).
A magnet just stuck on the end of a filter? I don't think it would do much. The oil passes into the filter through the 6 or 8 holes in the endcap, goes through the element, and comes out the large hole in the center. There is very little oil flow at the end of the filter.