For sports, I know that a faster lens would usually be considered more important than image stabilization, but what if, after selecting your shutter speed, your camera is picking apertures that don't require a super-fast lens?
Here's my specific example: I bought a Sigma 70-200mm F2.8 lens for my D90 thinking that I would need the F2.8 in order to shoot at my daughter's Figure Skating club. I chose the Sigma over the Nikon 80-200mm F2.8 because the Sigma had full-time manual-focus and a built in auto-focus motor and was about $100 less expensive. The Nikon 70-200mm F2.8 with VR was well beyond my budget.
Now, having used the Sigma a few times, I am pretty happy with the shots I am getting. I set the camera for a shutter speed of 1/500s and ISO-800 and let the camera pick the aperture. However, it turns out that most of the shots I am taking end up with the camera choosing an aperture of F4 to F5.6 (the lighting in the rink must be better than I thought).
So, my question is, would I be better off returning the Sigma 70-200mm, and getting the much less expensive Nikon 70-300mm with VR (saving almost $500 that I could put into another lens or a tripod). The aperture range of 4.5-5.6 on the 70-300m is around what I am shooting at, plus the lens has VR (I know it doesn't help with moving subjects, but I am shooting hand held at 1/500s for the moment).
I could even bump the ISO up to 1600 to maintain or increase the shutter speed with the 70-300mm. On the other hand, in a different arena, the lighting might not be as good, and I'll be wishing I still had the F2.8.