Let me phrase it this way and see if you agree:
If you are using a bit as a tool of force to intimidate your horse into obeying, you are badly screwed up. If you are using it as a training tool to encourage your horse to obey until your horse knows enough to obey willingly, then that is OK.
In like manner, if you rely on using a crop all the time to get your horse to trot, you suck as a trainer. But you might use a crop as a training tool to teach the horse that life is better if you trot when asked politely.
Mia has some big training holes caused by irrational fear of almost everything. So it is fair to use tools like bits, legs, and slightly scary situations to train her to not be afraid. But it would be wrong to rely on inflicting pain to control her. Not only would that not work, but it would be horribly unfair to the horse. Training holes should be filled, not papered over.
Is that on track?
I think it depends on how you go about it. Training a horse, I guess is like teaching a person about a subject they know nothing about, except with the horse you have a completely different language and tool kit to use.
When I walk into the yard with a horse that has never been touched, it has its preconceptions about what is going on, perhaps, "I'm about to become food for this predatory, eyeball on the front of the head having, thing". In my head is a different preconception, mainly "you have no idea what I am going to be saying to you so Ill break it down to bite sized pieces and give you time and room to digest them". I ask for response A, if I don't get it, I ask a bit louder, if I still don't get it I ask louder again till I get it. That's the basic principal that operates throughout the horse's training. And I think that's probably what you are getting at, so Id agree with that. There are a bunch of variables, as always that requires one to modify approach, technique etc., but the basic principal is the same.
Where people are talking about bits and more force with training holes is, from the way I read it would be akin to : horse doesn't go in a, say snaffle, so whack a curb in its mouth and off you go without assessing what went wrong in the first place, where holes in the training might be, or holes in the handling, and do everything exactly the same as before. Then you will probably just be using increasing amounts of force and equipment that has a harder potential.
Perhaps there are a lot of people who do it that way, when I was a kid and learning it all that was generally the way here, horse doesn't respond well, make it through any means necessary without actually having a think about what was going on, I have seen some pretty hard men on horses. The hardest I ever heard about was a guy who used to be a stockman for my uncle. I didn't see this, I just heard about it, apparently one of his horses wouldn't stop and kept running through the bit and he had no idea how to deal with it so he made a barbed wire nose band and connected it to the rings of the bit. Absolutely horrific stuff, but he has even had to shoot horses he has been training because he has injured them so bad.
That's the kind of thing I think people mean when they talk about using force on training holes, maybe not that nasty, but along those lines- can't figure out how to fix it? Just bash away at it till it looks kinda fixed.