I've never understood the "shoulder width" instruction. I know very few good skiers, including instructors--and including most who advocate the "shoulder width" rule--who tend to ski with a stance anywhere near that wide.
It seems to me yet another example of traditional "conventional wisdom" that, upon closer examination, doesn't hold the water many believe it does. For example, when Michael Rogan and others say, "We start at feet about shoulder width apart because that's where they are naturally and that's where, on average, we're going to have our feet flat on the ground," I have to wonder if he's really thought about what he's saying. I suspect that the image in his mind is accurate--I support that, as a "starting point," a natural stance width similar to walking or just standing still is a good place to start. But I would hardly describe that as "shoulder width"! I would not say that Michael Rogan himself skis with what I would describe as a "shoulder-width" stance--not now, and not in 2005 (it is much narrower than that).
A natural, functional stance is important in skiing. What does that mean? Yes, it can vary between individuals, and equipment setup can necessitate discrepancies in stance width as the skier compensates for non-optimal canting and alignment. It can also vary with the conditions and specific needs anticipated in the near-term. (Skiing boxes and rails in the freestyle park, for example, calls for a significantly wider stance than many "traditional" alpine conditions and situations.) But in general, a "functional stance width" allows the legs to tip, flex and extend, and rotate independently of each other. More separation strengthens the mechanics of "independent leg rotation" for twisting the skis, but tends to make the movements harsh and lacking in subtlety, "touch," and finesse. Too wide also creates lateral (foot-to-foot) balance difficulty, and compromises fore-aft balance and pressure as well when the legs are rotated in the hip sockets (tending to cause one foot to be pressured aft and the other forward). Too narrow eliminates altogether the ability to turn the legs independently in the hip sockets, necessitating gross and unrefined upper body movements or blocking pole plants when the skis need to be turned or "steered."
"Functional" may seem like a vague cop-out, but it really is a better description than any specific measurement--hip width, shoulder width, inches, and so on. There is too much individual and situational variation for any particular measurement to apply universally.
But any way you look at it, it remains critically important to recognize what "stance" means in the first place. It is not--did I mention, NOT--an instruction of how you "should" ski. "Stance" shares the same etymological root as "static," and thus hardly describes the continuous and vigorous movements of skiing. As in tennis and virtually every other sport, "stance" refers to an attitude and position of optimal readiness to make the movements required. It is not, itself, those movements! A "perfect tennis stance," for example, might involve "feet shoulder width or more apart, ankles, knees, hips, and spine substantially flexed, balance on the balls of the feet, with hands and arms in front." But when, when actually playing tennis, do you actually "stand" that way? Right--never! It's how you stand in preparation for playing, as when waiting to receive a serve, to maximize your readiness. LIkewise, I've often said that, even if I had a "perfect" skiing stance and made a series of "perfect" turns, you'd never see me in the "position" of the perfect stance. Stance (readiness) influences and informs the movements we make, but it is not itself those movements. Indeed, the sole reason for a "good stance" is that it facilitates movements away from its static "position."
Stance is a critical concept. But it backfires when a misunderstanding causes us to try to "maintain" the characteristics of any static position. I once heard an instructor give someone the feedback that "you made some great turns there, with awesome skiing performance, but I noticed that your hips sometimes got behind your feet...." That statement suggests a critical misunderstanding of the concept of "stance" (regardless of where you think your hips "ought" to be in a good stance). A good stance amplifies our movement opportunities. Don't let a misunderstanding of "stance" become a handicap that restricts movement options instead!