One thing I have learned over the years of dealing with high powered WBs (for dressage) is that you do not, under any circumstances, let go.
Yes, I get dragged, yes the horse gets jerked on when it happens. But there are no injuries, which is the main thing.
Lunging, leading, riding, you do not let go.
I think it is different in many other disciplines and with different horses but I would rather wrench my arm, get dirt down my pants and have a horse with a sore palette than deal with broken limbs or trying to catch the buggered horse for 45 minutes (yes it has happened in a 80'x200' enclosed arena with about 4 people).
They say money doesn't buy happiness -- well happiness doesn't buy horses!