To me, IS is a great feature that a larger aperture cannot provide. Imagine shooting a moving train in a metro station like the one in Washington DC. You want the train to be blurry while keeping the unique architecture sharp. F2.8 is not gonna help. You need that fantastic IS. I had a 16-35 I, and seldom shot at 2.8. For low light shooting 2.8 is just not fast enough.
A tripod can do the same thing. However - a tripod cannot get you an extra stop of light.