I would agree with you if the magnet was strong. The magnet I used is a Mercedes magnet and now recommended by Mercedes. It is about the size of a business card and about as weak as a kitchen magnet.
I placed the magnet at the bottom corner of the pan (near the drain plug). Avoid placing the magnet near the inlet of the trans filter to avoid the filter from sucking up the muck.
My Chevy Astro came with a transmission pan magnet from the factory. The magnet does a great job of collecting the muck. When changing the trans fluid and filter, I always clean the magnet which is very filthy.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles