Magnefine is way over priced for the teeny weeny filter.
It offers alot of bang for that buck.
I always cut mine open at regular replacement intervals and find the magnet doing what it is designed to do--pull out ferrous metals. Mine are located in the return line from the cooler and all of this fluid must pass in close proximity to that magnet,,,much closer proximity than a magnet in a pan...IMO.
Besides, it saves me having to drop the pan as often to clean that magnet.
The Magnefine also has a traditional filter element and this captures non-ferrous contaminants. Lets face it, the typical AT filter inside the transmission is not a small micron design. The engineers do not think it is necessary so the Magnefine is already offering several times the filtration capability.
You do bring up a valid point, it is classified by the World Governing Automatic Transmission Filter Council as "teeny-weeny"
and therefore needs to be changed out about every 15,000 miles IIRC.
plus its small design is less likely to get smashed by a rock in off-pavement applications.