HA,
I have seen it in practice and it has way more utility than you suggest. There is no direction change of his upper body-and that's the point! He snakes his skis through quick and tricky surfaces while avoiding having to bail from his chosen line. Now, imagine the same run, but now with trees, shrubs, logs and rocks demanding you hold the same line (typical expert tree runs in VT, NH, and gnarly western Mountains Like Red Mountain and Whitewater in BC), your options become limited, but the ability to get your skis to slither and negotiate tight/ trough-strewn terrain while keeping your upper body moving in a more direct line allows for a fast commanding expert way to ski such terrain.
See, what you point out as it's weakness-the ability to quickly turn the skis/ guide the skis (in very trick spaces) without changing directions is what gives this move it's utility. Sometimes you want to (or have to!) keep your body moving straight or in a corridor (not just tight trees and chutes, but think about late spring skiing in trees or tight bumps were all but one safe line has melted out), but what's happening at your feet-level (obstacles, troughs, bumps, stumps, Liftline poles and cement platforms, etc) makes that a difficult proposition. This move is a way to rectify that. And it's one that allows the skier to keep his speed up, and not use hard braking movements.
Take the 1st video above...aside from looking kind of cool, he was able to keep his speed and direct line through what was a succession of deep troughs that would have otherwise required deflection turns (wc mogul style), edge sets, or taking another line. Instead, he slithered his skis right through those sections and then resumed his more conventional attack on his chosen line.
Honestly now, how many of us on this board could ski that same run as well and with as much energy (or style) as the Japanese skier featured? You don't think that maybe he found utility in that swivel move that enabled him to ski the line of his choosing?
H-A, this move is undoubtedly from a past era, but that to me is hardly a reason to decry anyone wanting to learn it. Modern tools are great, but you ski enough varied terrain and you learn that sidecut isn't always your friend and sometimes something else might be needed.
As has been said In many, many threads and forums for years: DYNAMIC BALANCE is the ultimate ski tool-it connects the modern and the premodern. And, the Japanese skier in the first video is a good example of this (actually, so is the yodeling Austrian skier in the second video!). All of his skiing happens from a great centered and re-centering balance from his feet up, which makes both his swivel turns and his mostly very modern rounded bump turns so effective-and it is why he is able to move easily from one type of move to the next.
Max, I'm sure the femurs are involved, and of course it involved pivoting, but I am not sure it is so simple (and certainly not so simple to bust it our where this skier does, maintain your centered balance and chosen trajectory). I think there is some foot and ankle steering as well. And this move is seldom done as 'one-slither turn' but usually (as in this video) done as a set of turns. That level of Pelvic-femoral rapid fire coordination is a fairly challenging kinesthetic skill. Is it an easy skill to acquire the ability to keep a floating edgeless ski, with perfectly centered and calm balance in in steep moguls? There is a certain level of composure required to pulling this off in challenging terrain that is also intriguing.
Oh, and yeah, the fact that it is pretty freaky-stylie is part of the appeal to me. But as you all know, I'm very upfront about the motivational aspects of aesthetically pleasing skiing. And you also all know, I don't believe I'm alone in this.