We used magnets throughout food processing systems. Anything other than the fuzzy sludge (trace amounts of fine metal shavings) was cause for concern. Metal to metal contact in any system is gonna wear. Ferrous will of course be attracted by magnets, other materials will not.This ^
They only trap ferrous metal. I typically get some black sludge stuck to my magnets, but that's it. Under a microscope that sludge probably looks pretty nasty though, rough and ragged particles. Since they're more likely to cause damage to other surfaces than soft metals I'd rather they not be circulating in any amount. Anything non-ferrous I have to rely on the filter to catch.
This is all my personal preference/opinion of course. I'm not going to make up numbers to try and push my point. I truly despise people who make up lies to push their own agenda/opinion/politics so I won't do it.
For the record I also like to put magnets inside the fuel tank near the pump pickup to catch small ferrous particles before they can get into the fuel system I use magnets out of old computer hard disk drives. Don't listen to anyone claiming the magnets change the fuel in any way (yes I've seen products that claim that), they're just there to catch hard ferrous particles and sooth my mild paranoia, nothing more.