Silver - You may be correct insofar as the non-use of the 'rotary' function because it's Not universal. So for teaching purposes the demonstrator may choose not to use it.
Curtis - You may also be correct that the rotary function may obscure the actual tying process. Not sure I agree with the non-use when the 3 strands were tied in. I actually think the rotary function would work well. I also think the use of the rotary function works okay when winding a weak quill like peacock. However, I tend not to use that function when tying a Quill Gordon, for example. Guess I'm too lazy to use it.
But with a streamer like the Grey Ghost on a 10xl or 6xl hook, I certainly would use the rotary function for the floss and the ribbing. Also, I'd expect the demonstrator to explain why he or she chose to use or not use that function of the vise.
I just found the demonstrations interesting from this POV. And the entire presentation by the two tiers was informative too.