If you ignore the boot flex and technique issues in order to get to the general principals at work, based on my admittedly limited knowledge and experience I believe that if you reduce the forward lean on your boots to provide a more upright stance it allows you to support your weight on your skeleton more and consequently you need to use your muscles less, and therefore can ski with less fatigue.
On the other hand, I believe that due to the physiology of the knee and the forces at work when skiing hard in various conditions and terrain that a more upright stance results in a substantially higher likelihood that you will injure your knees.
Bottom Line: A bent knee is more solid and stable than a straight knee, and the straighter it is the more susceptible it is to a twisting or hyperextension injury.
Acknowledging that the forward lean numbers manufacturers use are not comparable across brands (just like flex numbers), for me the noticeable reduction or increase in fatigue and stability is at about 15 degrees forward lean.
Anyone else have any information or insight on this issue?
Disclaimer: I am not a doctor, but I play one on TV.