I know, oh I know, I am going to get a ration of crap for saying this, but why would anyone force their feet apart? I started skiing in 1965 and was mentored by a guy by the name of Frank Covino, a close friend and fellow ski instructor of Stein Eriksen. It is the most unnatural feeling to me to open my stance. All of my balance seems to disappear.
It's a trade-off between stability and agility.
Do a straight line speed run on the groomers into a tuck and I'm willing to bet that you open your stance for stability despite all the decades of feet together ingraining. In that situation what is needed is extra stability. (If you are skiing feet locked together at 60 mph... well, it's been nice knowing you.)
Now look at McGlashan - in the bumps, stance is narrow as agility (and having the skis experience the same slope conditions) is important. On the groomers in wider turns his stance is wider - look at the stance at transition, not when he's inclined and the skis are vertically separated. In short turns his stance is somewhere in between.
Now compare JB in wider turns vs bumps. Same thing.
Most important thing is not the width of the stance - which should vary for the situation - but that the legs act independently. Resting the inside foot lazily on the outside foot is not the optimal way and will limit performance.