You got a fair idea bsms, I think the old-timers that originally created the true spade bit, made it intimidating, and "scary" to the novice, uneducated rider. So they don't just pop it in their horse's mouth to look cool. But uf used correctly, on a horse that is trained to carry it. It is one of the mildest bits around. It doesn't really fit into the leverage category of bits. Leverage bits work off the curb strap, the spade is a category in itself. It works like no other bit in the transfer of signals, with vibrations, slight shifts and movement of the chains to many different points in the horses mouth. And the signals are never painful if used correctly.
~Started young horses in Bosca te Ador, unto the two rein the old Spanish spade, brought them along with two hands that were gentle. Some fine reining horses as ever were made~