I can't say I agree with this at all.
As long as you can maintain balance while you are pivoting there should not be a problem.
Balance is balance , it does not matter whether the body is stationary or rotating , as long as the center of gravity is kept centered.
The only way the stance will be compromised is if the opponent manages to get to the side and you fail to pivot to follow him and face his centerline.
The other issue with not moving the legs is that in trying to generate power you are leaving part of your body mass out of the equation.
A bit hard to use your whole body mass in striking or other actions when your legs aren't coming along for the ride.
One of the principles for generating force in Wing Chun is that in order to generate force , all the force vectors must be going in the same direction you are trying to project your force.
Good response. I also can't agree with this "no pivoting" concept...probably because I can't even begin to visualize how he does his chum kiu without it. I'm going through the steps in my head: you do your double jam sao, then double lan sao...and then you HAVE to pivot. What is he saying, that he turns his body only from the waist up? That will leave you twisted and VERY easy to knock off balance.