I agree with Robin, 100%.
The rotary skill encompasses all the movements and physical principles we can use to control the direction our skis point.
Skiers DO try to control the direction their skis point--often because they HAVE to, other times because they know no other way. Becoming SKILLFUL in how we do this is quite obviously important in becoming a good skier! How could anyone argue otherwise?
Rotary skill, and applying rotary skill, is hardly the same as "grossly twisting the skis into a skid"--although it is how we do that when we need to.
There are many ways we can cause our skis to turn, involving a variety of biomechanical principles. Rotation, counter-rotation, independent leg steering, blocking pole plant--they all involve movement, of course, so they all affect the position of the body. They all affect STANCE. Stance affects everything else--our ability to control edge angles, pressure, and even to apply the rotary skill! And stance affects safety, and the body's ability to withstand the potentially dangerous stresses of skiing.
Furthermore, the various rotary mechanisms are not equivalent in how they turn the skis. Some (rotation, blocking pole plant, and their various subforms) introduce "angular momentum"--they throw the skier into a spin, which he/she must then deal with.
Most (all except independent leg steering) involve the upper body, so they affect stance, balance, and all other movements, and detract from the upper body's ability to do other important things. Most (again, all except leg steering) are gross and imprecise, techniques best used to throw the skis quickly or powerfully in some new direction--important in "emergencies," but ill-suited to steering precise turn shapes.
The one rotary mechanism critical to today's high-performance, precisely shaped turns--independent leg steering--is extremely misunderstood. It is the least intuitive, the least likely to occur in an "unschooled" skier--although the very short skis some ski schools use for beginning skiers do help with this. I'd say that most instructors are fuzzy on it, although I've seen great improvement in that regard in the past couple years. And most skiers neither understand nor demonstrate any signficant skill with this critical tool.
So--for safety, control, precision, and above all maximum enjoyment--we MUST learn to apply rotary mechanisms skillfully and appropriately. Those who cannot are hacks--or novices--pure and simple! And those who proclaim otherwise are not thinking clearly.
None of this argument should suggest that good turns all involve twisting the skis with powerful, or even subtle, rotary movements. Nothing I have said conflicts with the fact that today's skis can do amazing, and fun, things when just tipped and pressured to bend their sharp steel edges into an arc. And, contrary to popular opinion, even powerful rotary movements are not necessarily incompatible with carved turns.
It's simply a question of developing skill--something you cannot be an "expert" without!