FYI-I don't even know why all companies are marketing these stiff boots for snowmobiling. I know that flexiblity is everything when riding. I know a guy that had a pair last year and came off of a small drift and hit another and busted his leg in multiple places just above that boot line. You legs are made to take shock like that, that is why your knees and ankles bend, they act just like a suspension. Now take out the movement of the ankle and you lose that. Try taking out a pivot point on the suspension of your sled and see what happens! Just my .02 I would try to stay away from a stiff boot like that, especially for agressive riding.
i ride dirtbikes and all moto boots are stiff, i understand what your sayins about joints but they only bend so far and i think without a stiff enough boot its way easier to shatter an ankle than snap the bone in your leg. i do agree it would be alot harder to ride a sled with stiff boots though.