So you still get pull backs and require a knife to free them, and in 7 years you've had to cut two or three ropes in situation where a horse could have been injured (or died if it had broken it's neck) - for me that is far too great a risk for horses that are my friends and ones I keep for their whole lives.
Regardless, you can easily see that the risk of a serious injury to the horses he handles from pulling back is actually nearly as small as their risk of being injured by a lightning strike.
Now, I say that tongue-in-cheek, but you get the drift. In my book, the risk of a horse being injured from pulling back, especially after being taught he cannot break free, is much lower than the risk of his being injured, or someone near being injured, as a result of the horse breaking free. The worst injury I have ever witnessed from a horse pulling back that could not break free was bloody gums, when a horse that had been allowed to break free in the past pulled, then lunged forward into a wire fence, hitting her muzzle in the fence. That was the second to last time she ever pulled back.
The last time she pulled, she hauled back on the lead, was just about to lunge forward, when I actually saw the recognition of the situation in her eyes and the change of her mind, and she simply relaxed and stepped forward. Never pulled again. Became a much safer horse to be around.
Neither have I ever witnessed a serious injury to a horse from the horse breaking free, but I have had to chase a couple that caused dangerous situations for themselves and some bystanders as they pushed their way through other horses while dragging a hitching post around, and I have been left afoot a time or two, by horses that broke a lead or snap.
Not saying your way is wrong for your situation. But your way is wrong for mine. Proving either way right or wrong with real numbers and evidence is futile. It's simply a choice.
As for me, I'd rather deal with a young horse pulling a couple times and done with it, than to have to deal with the possibility of a horse pulling and breaking free at a critical time later on.