IS helps reduce the effects of camera shake (which becomes more problematic as focal length increases), but does nothing to stop subject motion. When you're shooting sports you need to use fast shutter speeds to stop the players' motion - those fast shutter speeds also eliminate the effect of camera shake.


So, if you shoot exclusively human action sports, I'd say IS will not help much or at all (I say 'human action sports' because for motor sports, e.g. auto racing, slower shutter speeds and panning are used, and IS will help a lot for that).