Actually, I think it's not a bad idea -- all it takes is for one overflow especially from a second-floor toilet to make a homeowner want to take steps to prevent it from ever happening again.
I wonder if it would actually prevent an overflow, though, since it simply interrupts the fill water if the bowl water gets too high. Seems to me it's the water already in the tank, not the "new" fill water coming in, which you'd want to stop immediately if the toilet was about to overflow...