Yup... If I'm not mistaken its more than just prevent the bassdrum and hihat stand from "creeping" forward. It's also to prevent the bass spurs from scratching the flooring, as well as improving (slightly) the acoustics of the area.
Without the rug or carpet, you'll be hitting your snare (or tom, cymbals or anything) and getting "bounce" off the floor you're on. Imagine trying to mic your kit up that way... :S