A beginner cross-country skier expressed concern about trying to control skinny skating skis on a downhill, especially if they were only on one ski, as he had seen some faster skiers doing.
Don't equate stability with control. This goes along with assuming that a wide ski will be better than a narrow one, but can appear as a separate problem to keep a beginner or intermediate skier from progressing.
I haven't been to a lift-served ski area since the late 1990s, but I'd be surprised if the Flying Starfish has become extinct. Flying starfish are skiers who only learned to control speed by making a wedge (pizza). When the wedge straightens out, accidentally or on purpose, the skier ends up in a wide stance that feels very stable laterally, but strands them with their center of mass hung between the skis. Their arms are usually held out from their sides at a downward angle. They manage to change direction somehow, but not in quick, linked turns.
The cross-country Flying Starfish comes in two basic forms. The touring starfish is just a version of the alpine starfish. It originates in the same insecurity, made worse by the detached heel of a Nordic binding, and the less substantial feeling of the gear overall. Whether you've been on alpine skis or not, cross-country skis seem like a really stupid invention when they start to accelerate under the influence of gravity and you don't know how to make them behave. Everyone tries to make that wedge, but that often leads to the wide stance as the skier bombs out of control, usually to end in a crater.
The ski-skater's Flying Starfish is also born of inexperience. The skating starfish is usually doing a V-1, which has a dominant and a non-dominant side, so both poles hit the snow at the same time as the right or left ski, depending on the individual's timing and whether they are right or left handed. To many skiers, a V-1 is skating. It's a visually obvious technique that's easy to grasp, but harder to understand and truly master. The starfish V-1 is also the common technique of sklassical skiers who like to mix techniques on their touring gear. It's the mythical "combi."
The skating starfish holds the arms high, especially if the skier has gotten actual skating gear, with longer poles designed for the skate technique. Rather than keeping the hands close to the body (and face), the starfish holds the arms wide in imitation of the wide stance of the legs. It can even happen to experienced skiers when they are tired.
On a climb, particularly a long climb, most mere mortals will use a V-1 or even a diagonal V skate, which does require placing the skis in a wider V, and making sure that the poles stay clear of them. That's the entry by which a tired skier can fall into the starfish position by continuing to plod after the grade eases. If you remember to tighten everything back up, it is actually more efficient and speeds whatever recovery you're going to get.
On a descent, beginner and intermediate skiers may revert to the basics, trying to rely on the snowplow to keep speed in check. Note: the best way to control speed is not to get it in the first place. If a descent looks intimidating, move to the wedge early. You can stand still in the wedge at the top of the slope and then let up very gradually until the skis start to move, or you can let the skis run just a little and try to put the brakes on smoothly, but what often happens then is that you either stop and start in a jerky progression, or you lose control of the wedge, the skis straighten, and you're the Dive Bombing Starfish.
On touring center trails, especially when the loose snow on the surface has been ground up and combed out by a grooming machine, skiers descending in a wedge will scrape all of the loose snow out of the middle, creating a dandy bobsled run for anyone who comes along after them. Learning to ski on one ski at a time will get you down that luge course with a modicum of control. If you happen to get there while the granular snow is still well distributed over the whole width of the trail, being on one ski at a time will save that snow for skiers who come along later. They may never know to be grateful, but you will have made the world a better place. Take satisfaction in that.
Descending on one ski at a time doesn't have to be much faster than plowing down in a wedge. You simply continue the rhythm of skiing on the flats, but you set each ski at an angle across the slope, and don't stay on it for long. If the ski you're on starts to go in a direction you don't like, your other ski is available to set on a new course. If you were on both skis, you'd have to shift your weight to one ski anyway to free up the other one. It's better to get used to being on one ski at a time, most of the time. You can even be in the wedge stance, though not a really wide one, and just weight one ski at a time to set up a turning rhythm that also controls speed.