I think they are a great idea. I've run them on everything I've owned for in excess of 10 years. I use neodynium magnets that were scrap from a job I used to do, though. The soft metals are irrelevant as they will not be hard enough to cause any engine wear, but here's a list of the metals that are REALLY nice to remove that a magnet will catch; Rings (chromemolly, iron) Timing gears (similar to tool steel) Cylinder bores (iron EXTREMELY abrasive metal) and a handful of other less wearing materials from valve train etc.
they are a really good idea on hydraulic systems, too. Hydraulic filters don't filter very well.
I don't remember seeing a magnet on my drain plug, but here's a simple question; what can it hurt? What can it help? The only non ferrous wear metals I am aware of in these engines is the babbit from the crank, rod, and cam bearings. This metal is simply not hard enough to cause any wear.
To clarify, magnets will not only remove iron, but any metal that HAS iron in it. Which includes all the hard wear metals in the engine. The oil filter will catch the vast majority of it, but the super fines might sneak through. A magnet will trap them.
Ha ha ha! MAGCRAFT® Neodymium Magnets
same type I use! From the ad banner at the top of this thread. Probably a lot cheaper than the "filter mag" units.