Mac, you seem to be blending two different issues. The tennis
ball/sytrofoam/toilet paper inside the goblet is doing a different job from
the string steady. The latter is strictly for controlling deflection and
vibration in a long thin stem. One of the former devices is still needed to
stabilize the bowl.

BTW, IMO, it would not be as easy or effective to run roller blade wheels
against a very thin stem as string.