...It's not for everyone and I will never try to convince everyone that this is the only way to train horses...
More than one way to train a horse? THAT'S a radical thought in modern horsemanship! Odd thing is, from what I've read, there seems to be at least a dozen "only ways"...
Your quote from Gwen Turnbull-Weaver:
"The horse trained and ridden in a leverage bit is not taught to carry the bit or have any sensitivity to its shape or configuration. They most often just respond to the curb strap pressure. A rider’s goal when using a leverage bit is to engage the curb strap as quickly as possible to achieve the expected results, that is to stop or at least slow down."
Hmmmm....the only horse I've used a curb bit on is Mia. I grant she does NOT like having the curb strap engage. However, as I have it adjusted now, it takes about 60 deg of pull for the curb strap to tighten. That is ample warning, and she increasingly responds before the curb strap comes into play. Thinking about our last week of riding, there is only one time I think it might have engaged, and then it would have been from a flick of the wrist - so maybe a quarter second of engagement, and maybe not then - I didn't flick very hard. (Someone went by on a bicycle with a tiny two-stroke engine. It had a weird whiny noise and Mia jumped. I flicked my wrist, she stopped, and I scratched her neck until the bad thing went away).
I think the proof of a bit for a given horse is their behavior. Mia felt she had the option of ignoring a snaffle, and that led to nervousness and head tossing. She doesn't feel she can ignore the curb, so she accepts the idea that the rider is in control - and that calms her down. And that calms me down, and that calms her down...and so on. When a horse is interested and relaxed at the same time, how can anyone argue the bit is harsh? With Mia, a fat, O-ring French-link snaffle is a harsher bit than the curb, because she will fight the snaffle and relax in the curb.
I will never have the patience, skill or use for a spade bit. However, it is obvious some do, with impressive results from the horse. It seems to me the horse is the ultimate judge.