If your filter is doing what it should, it's completely unnecessary

Particles less than 10 microns is not going to cause any harm to the motor...if you want to filter out particles in the 1-25 micron range, use a bypass filter. It will get the non-ferrous particles too
It won't hurt anything though...it also is not going to live up to some of the claims the web site makes. Basically this is a "snake oil" like product. If you want to use magnets on your filter, remove the neodymium magnets (same ones the FilterMag uses) from a busted hard drive or get some on e-Bay and stick to the side of your filter.