I agree that I'm very against using backing as a punishment, and have been in some dangerous situations with horses due to that type of training. Just today I was advised to use backing as a way to teach a horse to stand still for mounting. I had a friend who bought a horse someone did that with. When she first got him, he'd wait until you were swinging your leg over and then back up really fast, which put the rider up on his neck or over his shoulder. It was a really hard habit to break.
I've been on a horse that backed into traffic, another that backed toward a ditch, into a barbed wire fence, and my friend's horse backed through a line of other horses when he got scared. They all threw themselves into reverse when scared. It's very difficult to control the direction or speed of a horse that is backing up rapidly, and they can easily slip.
@DollyandAya, a horse that is backing rapidly is basically balancing on the two hind legs, and if you pull on one rein you can easily cause them to slip and fall down on their side, or else pull them over backward. You really want to keep the reins loose, and relax, and hopefully they will break out of it long enough to get their weight forward so you can begin to cue them again.
The only time I will use backing as a punishment (sort of) is when a horse is worked up and barging on the lead, and I will make the horse go back so they are not forging in front of me. But that's only a couple steps, and more of a way to tell them not to rush and push forward.
I think it's important to teach the rein back thoughtfully. If you're planning to do dressage, or to use it as a way to teach sliding stops, or if you want the horse to be careful and slow, there are many right ways to do it depending on what the horse will be used for. For example, you might want to teach a horse that is going to be driven that backing up should be done slowly, with each step carefully placed. That's my goal currently, because my pony is going to be driving, and I want to be able to control the vehicle's direction if I have to back her out of a situation where she is stuck. I'm training her to back up from a voice cue, which she's doing very well so far.
Here's an example of the balance and speed of a horse rushing back. Not something you want as a horse's "go to" move.