Big fan of Fairbairn myself, he epitomised the KISS principles, the simpler something is, the easier it is to learn and it's easily remembered.
This is especially true in times of high stress when the body goes into flight or flight mode and isn't really capable of fine motor movements, only gross motor movement.
If it didn't work, or there were doubts, it wasn't taught. Simple as that, he taught only what worked.
Granted, his system was not pretty, it wasn't really designed for restraint either but it was born out of conflict on the streets of Shanghai and later during WWII. It simply had to work.