I think we can all agree that any bit is only as harsh as a rider’s hands. So the question is back to - why a shank? Many of the responses so far have been that high-end western horses are so well trained that they can be ridden on the buckle. So I guess the (naive?) question remains - if they are that well trained to the slightest body weight cue, then why not on the buckle on a snaffle? If your horse stops when you sit deep without touching the reins, why do you need a long shank in his mouth?
Soooo not accusing anyone of anything. Just trying to learn! If the answer is, as it seems to be, that they are super refined and highly tuned and need no contact, then why the relatively high-test bit? (Full disclosure - I also wonder why dressage horses need to go in a double bridle per the rules even if the individual doesn’t require it.)