It’s widely believed that mounting the bindings further apart “should” provide a more stable stance, but maybe not.
Clearly, a wide stance on solid ground is more stable than a narrow one. But standing on your ski is not the same as standing on solid ground. On a ski, your feet are clamped into bindings that are bolted to the ski. This is a really solid connection regardless of foot spacing. Furthermore, your ski is in water which makes standing on it more like standing in the middle of a teeter totter than on solid ground.
If you were standing balanced in the middle of a teeter-totter, would it be easier to rock the ends up and down if your feet were tight together or spread wide apart? Clearly, a wider stance gives you more leverage. But this wide stance on your short ski makes it easier to overload the tip or tail when (not if) you get out of balance on your ski.
And when you widen your stance on a ski, you’re not staying balanced over the middle the way you would on a teeter totter; you’re just moving your back foot towards the tail creating an imbalance. The short, skinny little tail doesn’t provide much support. So the ski will tend to ride tip-high with a deep tail plowing through the water.
Standing further behind your front foot also makes it more likely you’ll get rocked back onto your rear foot, driving the tail even deeper during the cut and/or preturn making the ski slow and unresponsive. Worse, this overload and easily forced tail exposes you to over-rotations and tip-grabs around the buoys.
The ski doesn’t work properly with too much weight riding on the tail. So an essential skill for any good skier is the ability to ride the ski’s tip around turns with most of their weight over their front foot. The ski can’t turn properly any other way. Think “tip to rip.” The further back the rear binding is mounted, the harder it becomes to move up over your front foot. And trying to make a ski turn from the back seat builds bad habits and delivers inconsistent results because the ski isn’t designed to be used this way.
One such bad habit is using your rear leg to drive the tail around onside (heal-side) turns. The further back the rear binding, the easier it is to over-rely on this bad habit. And it’s a bad habit because you can only do it on your heal side of the course. If you don’t ride the ski’s tip around turns the same way on both sides of the course, your ski’s setup will have to be compromised because you can’t set it up to do two different things perfectly—effectively lowering your ski’s performance potential.
It may seem counter-intuitive at first, but the closer your feet are together, the more stable the ski’s behavior will be, the more steady your balance will be, the better your ski’s setup can be, and the easier it will be to get up over your front foot so the ski can behave the way it was designed to work best.
There are only a few legitimate reasons to move the back binding rearward, but those are special cases that usually involve compromises or Band-Aides on bad habits. For the vast majority of skiers, the closer the feet are together, the better balance and performance will be.