I always use a magnet on the oilfilter and wanted to know if it makes sense and therefore opened my used filter with a saw.
The Mazda-OEM-filter was used approx. 6.300 mls (RX-8 from 2005, 231 hp, odometer 26.000 mls, always premixed and with 2 mm. restrictor in OMP vac. line)
I was glad to see that the accumulation of metal particles was very minimal, it was more or less a kind of "powder/dust", concentrated around the magnetic poles with no grain or chips etc. The latter would have been a huge problem of course.
I know the existence of magnetic drain plugs but since I use the Pela oil extractor for all my cars this is a better option with more surface for separation.

 

Yup, oil filter magnets are one of the best ways to keep metal particulates out of the oil system that aren't already stopped by the filter, since the floating metal gets removed with each filter change. Also a lot of surface area as you note.