I guess it kind of depends upon the level of the skier, doesn't it?
If a skier were a relative beginner, and the width of the (braking) wedge they used at times was shoulder width or wider, that is a possibility. For any wedge to be that wide would certainly be of that variety!
But for a intermediate or advanced skier, it seems to me that a shoulder width stance would be quite difficult to maintain or control with any degree of accuracy. And I can't say that I have heard of that being a serious suggestion, other than perhaps during an exercise or drill. Too wide of a stance can be quite limiting as to the range of movements which could be achieved from it. If other sport comparisons are done, those where any sort of impact is expected will occasionally show a wide stance. But most which are movement oriented, without the need to brace for an impact, tend to be much narrower.
Even the idea of a "hip width" is wider than I believe most skiers are comfortable in. But the width issue is one which is personal to each skier. Every person will determine where they are comfortable, based upon skill level, experience, and desired outcome. The term "functional" is useful, but it offers no parameters or guidelines to describe what is "functional". And though I can understand SureValla's idea of 'functional' in Karate, when I studied, I learned many different stances, from the wide as he has described, to 'cat stance', which is predominantly one footed and quite narrow.
I personally encourage people to ski in a somewhat narrower stance, or as narrow as the above mentioned factors allow, without ever letting the boots/feet/legs actually touch each other. For at that point, that touching become restrictive and limiting to balance, edge control and the ability to turn the legs/skis without involving the torso.
So once again, just like so much in skiing, too much or too little is not the norm or optimum, but rather effective, efficient skiing happens somewhere in the mid-range of these extremes.