I'm posing this question because all my life, I assumed that desensitizing or "sacking out" was an acceptable facet of training. This summer I apprenticed under a trainer that said that desensitization was unacceptable. She told me that when you sack out a horse you are actually numbing them out. They do not learn to trust you and they fade back into their "survival brain." Sacking out can actually harm your relationship with your horse. She believes that your horse should trust you enough to where you can do literally ANYTHING to and around your horse, and their trust will override their wary instincts. She also says you can't desensitize your horse to EVERYTHING they could EVER encounter that would be scary, so its pretty much a waste of time to begin with.
I respect this point of view, but I still see the merit in desensitization training if done properly.
My question is: do you believe that sacking out is an acceptable outlet for getting your horse used to new and scary situations/objects/whatever?