You wouldn't feel anything from a magnet in your pocket. The point of the implant is that it is small and touching a lot of your nerves, so the field can be felt.
If you kept the magnet in your pocket, you can take it out whenever you want to test a magnetic field. That seems about 1000x as practical as implanting it via amateur surgery.
http://www.theverge.com/2012/8/8/3177438/cyborg-america-biohackers-grinders-body-hackers