everything nomensteven said, as well as rails and swing weight.
With rails, you're skis and you are directly over the rail in the right part of the ski, so it can help stability.
With spinning, there is equal distance between boot and tip, as boot and tail, so you have the least amount of swing weight (ease of the ski to spin in the air).
But it probably doesn't help as much with anything but skiing switch.
And if you ride other parts of the hill your tips will dive, and there will be a lot more tail to have to turn.
So it's not good for powder or steeps.