^^ Precisely.
You want that crud getting caught in such as way as it is easily removed. Put a strong magnet on/around your oil filter or very close to your drain plug, so the materials it catches are easily removed when you remove your filter, or drain your oil.
Ultimately, while this Magnafilter may work, at least at first, I see it as ultimately becoming a problem as the stuff builds up and starts to either form a bottleneck, or breaks loose and overloads your filter suddenly...
or breaks loose at an inopportune time, such as when your filter is in bypass. Imagine a glob of ferrous metal suddenly circulating through your engine. Not good.