A couple of points:
First off, regarding IS with a tripod, there are a couple scenarios. First is one where you want a longer shutter speed and are shooting "hands off", like the self-timer or mirror lock-up. What can happen is that as your are waiting for the shutter release, the lens IS can actually de-activate, which will cause the focus to "slump". It will vary with the lens, but I know that with a long focal length this can be bad!
With other shooting scenarios, some people describe the IS as "vibrating" when there is no actual camera/lens movement.
However, there are occasions when I'm shooting with a long focal length, using a tripod but doing "hands-on" shooting, as I've done with some distant wildlife. In those cases, you see camera shake very clearly, and IS helps!
As to motorsports, Tanya, you could benefit from learning about Panning, where you keep the vehicle in focus, using and aperture such as f/8, but you have your shutter speed slowed down as you "follow" the vehicle (AI Servo AF). As a result, you see the "blurry" background but a nice sharp/crisp vehicle!