@elijah – well it is like saying why teach a child or a dog or anyone, why not just let them go through their entire lives doing whatever they want????? Then when the horse runs wild and tramples some human because it is not trained, people will say what a bad owner the poor creature had.
I am about the softest animal loving person you could ever meet. I have spent a large portion of my adult life (not to mention my income) rescuing animals. But I do believe in training and using whatever method is the most productive, because an untrained, undisciplined creature ends up being socially unacceptable and eventually KILLED. I also believe as do most professional trainers, that there is no one way that works for all so to immediately dismiss a method without knowing the history of the animal and the skill of the trainer is irresponsible and well ignorant.
Use the almighty google and see what it says about lunging horses and the use of the whip as a training tool. Or better yet, go out and take a green colt and give it a go, then tell me what you think.