I don’t necessarily sack out a horse as a major part of training, though if I’m using particular things to handle the horse, like a halter, a lead rope, a lariat, saddle, saddle pad etc, then I will let them get used to that. When I first learned to train horses, the good old Ozzy way (there’s nothing god about it in my opinion) I was taught to bag out a horse (sack out) by getting an old hessian feed bag, or an old saddle blanket, and gently whack the horse all over with it. Yet the horses I did that way, well they weren’t much good. After I learned how to do it properly, I stopped bothering doing it as a main thing. I get them used to what I need them used to in order to train them, I get them so they will accept me handling every part of their body, the rest comes with respect, trust and time, and the trainer being dead calm through everything. Having said that though I wouldn’t call sacking a horse out abuse, not by the wildest stretch of imagination.
A case in point would be the horse I’m riding in that picture to the left there <<<<. I never sacked her out, and was taking her out to do some lunging around some obstacles. I had to walk her between my uncle’s workshop and his truck, which he had the cab tipped up on, and was on top of the engine of, trying to do some mechanical work, at the time. Just as I was going by my uncle dropped a spanner and started swearing. The horse crapped herself and leaped away from the truck, as she landed the radio in the shed had the Morse code beeps of the news blast out, the horse spun round and leaped back at the truck with my uncle swearing. The whole time I stood calmly and just waited for her. She spun round back to me and stood facing me with her head down near my belly or chest chewing and relaxed, all of it happened in the space of half a second, and each time she went to take off she was watching me to see what I would do, I just stood there and waited for her to chill out, and she did; and she has been dead calm ever since. She is in no way dull or withdrawn and she isn’t bothered by much anymore, as long as I’m not.
So, I probably wouldn’t agree with your trainer saying sacking a horse out is a bad thing, though I would agree, to a degree, that you will never be able to desensitise them to everything they might be afraid of. I’d think of sacking out a horse as one of a number of tools you might have in your bag of tricks to train a horse, you can pull it out if you need it. The more important thing in my opinion is getting the horse’s respect and demonstrating to it that you are worth trusting by giving it a good and calm reliable leader it can follow. I you stay calm, even if the horse is about to blow, it will come round.