My thoughts behind it was,
1)I can remove the harness from the back seatbelt and then still use the rear seats
2) a 3 point holds you so much more tightly when braking hard allowing more control. Compared to standard belt.
3) I only use both belts on a track day for extra protection incase my harness mount fails, or in a crash with harness mounted below the 45 degrees to the seat harness holes, you can submarine under the lap section.
Fia standards for harness require them to be mounted parallel to the upper seat harness holds or maximum of 45. Due to the risk of submarining and back injuries.
Thats why the lower rear seat belt mounts are not the best idea. But it can be done.