If youre lunging your horse so much you have to worry about his spine because of a pound of line between you and him you are probably overdoing it and should get on and ride or try something else.
Kevin, I tried to search the Monty Roberts site for this great article, but their site is under construction or something. He does have a great movie clip that illustrates his point. All I found was this: Monty Roberts | Join Up Ask Monty
Please check it out and let us know what you think.
But, here is my attempt at his explanation: With a single lunge line, pressure is applied to the atlanto-axial joint (their poll) which starts the imbalance. Without outside contact (through side-reins or the outside long-line), the bend of the horse actually becomes almost counter to the circle you are lunging. This throws off the whole orthopedic movement of the horse. Really just the pound of line is enough to cause this imbalance. Even if single-line lunging is done minimally, it is still not favorable.
I agree, many problems can be resolved through just getting on. However, lunging young, untrained horses is a great training tool leading up to riding. Also, certain horses may benefit from lunging when they are coming back from injury, or just out of condition. Lunging horses properly can really help them balance their bodies before they have to juggle a rider. And, frankly, some in experienced riders may not ride well enough to get horses through those training periods. Lunging is a great skill.