two theories:
(1) solid single-ski balance is difficult to learn for classic -- requires going outside the comfort zone for most skiers -- take the risk of falling over to the outside of the track with nothing to catch you.
So people in their skiing time outside the lesson are unlikely to try things that will help them learn this critical balance. Therefore the lesson time will "add the most value" by helping learners focus on what's pretty hard for them to try on their own.
(2) It all depends on the skis and snow conditions. This theory is that people learn bodily moves best when they can _feel_ the benefit.
The main point of single-ski balance in classic striding is to get _grip_.
(a) If a learner is on waxable skis perhaps a little stiff for them, and where their current wax job is just barely working for the current snow conditions, then getting grip to push against the snow is tricky -- so solid single-ski balance is really critical.
Therefore when doing balance drills the skier will strongly notice when they're "getting it" and when they aren't. The instructor won't need to tell them: The snow will tell them. And their unconscious neuro-muscular controller will remember.
(b) If a learner is on waxless skis a bit too soft for them, on a warm afternoon with mushy snow, then getting grip is easy. Single-ski balance doesn't really matter for that skier in that situation. They can go thru the motions of three good balance drills, but after the lesson they're going to say to their friend: "I guess I got that balance thing, but I couldn't feel much difference." And their friend will say, "In those lessons they just teach you things for racers. Let me tell you the move that really works for me is ... "
(3) bonus theory:
Some people are just always going to be "shufflers". It doesn't matter what sequence you teach them.
Ken