Not 'smooth zoom' but rather to support smooth autofocusing during video shooting. The IS system on the STM version was updated to include automatic panning detection and 'dynamic IS' which is a more active implementation of IS that's automatically activated when the camera is in video mode (maybe only with the T4i, not sure). The STM version also focuses closer (about 4") - the maximum magnification went from 0.21x to 0.28x, so the STM version is pretty good for close-up shots of flowers, etc.
