My WP experience is somewhat limited, but if you are looking for a biomechanical answer, I can definately help you there. Ok, when we teach softness vertically, we are basically teaching to "give" to the pressure. This makes a very nice, light feel in the rider's hands, but the reality is, it isn't the best thing for the horse. (nor is tying, but we'll get to that).
Try this, stare straight ahead and drop your chin immediately to your neck. You will feel a bit uncomfortable right behind the jaw bone, and a bit stiff through your neck. This is basically what your horse does when the first thing taught is "give". The horse learns to give to escape the pressure. Going beyond the verticle would be known as hyperflexion. Is it soft in your hands, yes, but correct, no.
Now, think through the muscles in your back like you are stretching yourself from your tailbone to your shoulders. Follow this feel up through the shoulders, through the neck, and let your eyes drop towards the floor. What you should feel now is more of a stretch through the back of your neck that actually feels good. This is how to achieve that much desired headset, it comes from the back, not from the front. Notice how you are actually looser along the muscles in the front of the neck and the jaw feels loose. These are the same things we are looking for in the horse.
Both of those examples on the horse result in the horses head falling on the verticle, however, the first one, where the horse is only taught to be soft to the bit, is your "false" give.
I have done plenty of western events, from speed events to cow work, as well as hunters and am currently (and have been for some time) studying more classical dressage and biomechanical functions. This is what I find interesting in the show world. In human sports, proper posture is proper posture. There is an optimal physical condition whether you are a dancer, a swimmer, or a ball player. If your sport doesn't balance the muscles, then a good athlete does a cross training program, as the purpose of the muscles is to support the joints. Even though they all have different "disciplines", there is still only one human body, and anyone who is going to be successful in their sport for a long amount of time will keep their whole body conditioned, not just the muscles they use.
So why is it different for our equine athletes? Why is the western pleasure horse to have one posture, yet the dressage horse to have another? There is a correct way for the horses body to work. I know someone who has a WP horse that was at one time top 10 in the world, the horse is only about 12 yrs old, yet physically destroyed, is that how we want to label our world standard?
Often, we hear people say to get vertical flexion, and then push the horse into your hands to get "collection". I call this the crash test dummy theory. Do we need flexibility? Yes. Strength, yes. Condition, yes. However, your "collection" should go from back to front. Instead of breaking at c3 (typical with the "give" horse), the horse should actually be lifting through c6 and c7, elevating the front end, lifting behind the withers, bearing the weight towards the hind legs and releasing the topline, falling into the bit, with the bit being a piece of communication, more for listening than for telling.
Often, when training horses (I see this a lot in NH ironically), we look at the horse as something that we have to install buttons on. When I give pressure here, you release there, etc. However, sometimes this repitition is not the best thing for the horse, the horse learns to give despite the fact that it can be going against its proper biomechanical function.
I actually just posted a similar thread, but didn't get many responses, since teaching a horse correctly takes a large amount of time and skill, it sometimes is frowned upon by a time demanding society. My question was if was wrong to knowingly teach a horse something that could be detrimental to its body in the long run for the sake of moving it onto a riding career.
I know you weren't looking for a book, but I will recommend one anyway, its called "Tug of War: Classical vs. Modern Dressage" by Dr. Gerd Heuschmann. I know you aren't into dressage, but as I said, biomechanics are biomechanics, and the book goes into muscles and how the body is damaged by many training techniques.
I hope this answers your question, I can go more indepth if you like.