Ha! I remember a heated debate between myself and Skidude a few years ago about this very subject and I see we both have stuck to our convictions. While I have great respect for Skidude's knowledge I am afraid we will never agree on this topic.
As a boot's flex gets stiffer, the skiers movements must become more accurate.
This means flexion extension movements in the knees, hips, and arms must coordinate so as to maintain balance with less ankle flexion available.
As a boot's flex gets stiffer, proper alignment becomes more critical.
A softer flexing boot poorly aligned on the sagittal plane can be flexed one way or the other to compensate so the skier can balance effectively whereas, a stiffer boot will inhibit finding a balanced position without compensating with the other joints.
There are pros and cons to stiffer boots for sure, but to a skilled skier the benefits are quick energy transfer to the shovel and the tail of the ski with minimal movement.
Yes the ankle needs to be able to articulate to some degree inside the boot on all planes of motion. However minimal that movement the energy is then transferred to the ski when the movement is stopped by the boot shell. The firmer that connection the more accurate and instant the transfer becomes.
The shorter the range of flex needed to apply X amount of pressure to the ski's length the quicker the response. Conversely this can penalize the skier as well. Matching the right balance of flex and response is important for the kind of skiing one prefers.
Now there will probably be a few photos thrown up to demonstrate how maximum flexion is achieved by top ski racers but I would encourage you to look closely at camera angles to decide just how much the angles you see are lateral or forward!?
I will also argue that, while there are times we will flex a boot deeply, it is not necessary in every, or even most turns for a great turn to be achieved.
A good skier can slam into a mogul in a stiffer boot and absorb it just fine without flexing their ankles using accurate flexion of other joints.