Absolutely. In "many" situations it is. Like if the snow is deep, I want my skis to glide almost completely along the length. Not to mention when I want to carve. In bumps though, a lot of times I want them to be a bit more sideways although rarely 90 degrees. Maybe like between 0 to 40 degrees? The secret is being able to modulate between that 0-40 degrees at will.
The problem is, this skill is hard, if not impossible, to see from outside. You can point out things like "hey your inside ski is too forward, pull your inside foot back" and things like that for carving turns or just about any turns. But it's kinda hard to point out what's wrong when a person is bit jerky doing, say, switch pivot slips. It's mostly very subtle edge and pressure control that you probably don't see.
So I said "unexplained" up there, but at the same time didn't say those videos are bad because, I can't really explain it myself, haha. I just did a bunch of drills and "got" it. If you or anybody here have a silver bullet in that, please share. By the way, I love HH's phantom foot method and think it is kind of a silver bullet in teaching people high quality turns. My feeling, which could definitely be wrong, though, is he doesn't like to do pivot slip variations all that much, and I think they help a lot especially in bumps, but at the same time like I said above, I understand that making my clients do them like a football coach probably doesn't work if I were an instructor.