...There was a ridiculous post that said skis together results in the tails skidding and that the technique was inefficient. Completely wrong. A smooth carved turn is the result of proper edge control and weight distribution and not dependent on whether the skis are together or a few inches apart. A skier that skis with the legs together will always have the skis separate on the steep (off the fall line) and in high speed tight turns. Simply not possible to keep them together all the time.
Well, you can... but then you can't carve and will skid like crazy. I have seen people doing this, with the 'knees jammed together all the time' look. I'm not sure if someone was actually advocating that 20 or 30 years ago or they were just (poorly) emulating someone like Stein when they learned.
If you're zipperlining tight competition moguls you sort of end up looking like this -- there's no room to carve and you want both skis to follow the same path -- but on an open slope it's not a very efficient technique.
I'm not sure about anyone else, but by a "wide" stance I wouldn't normally mean wider than hip/shoulder width, unless you're talking about some kind of drill like cowboy turns. I really mean something closer to 'not having your feet jammed together as closely as possible'. The optimal width is going to depend on the skier, the terrain, the speed, the kind of turn, the kind of snow, etc.