I have heard these same sort of arguments made about mx boots. The theory is that with the armor stabilizing the foot, ankle and shin, that in the event of a crash it can simply transfer that "strain" further up your leg into the knee and hip.
I guess that optimum protection would dissipate the energy from the impact over the largest possible area. It may be a case of optimizing the entire protection "system" rather than optimizing the protection in a single area.
In my case, I have knee armor, but not braces. Not sure if that is enough, but it is what I use.