I use IS on the tripod all the time. Remember though that this is using a gimbal head with one hand on the lens and one on the camera, so the system is still detecting sufficient movement to be effective. If you doubt the effectiveness of IS in this situation, make this simple test.....
Focus on something and then make a concerted effort to hold the camera as still as you can. You will see the very slightest shimmer of the image in the viewfinder. Now half press the shutter button and as you feel the IS kick in you'll see the image become rock steady in the viewfinder.
Using a tripod with the head completely locked off and releasing the shutter with a remote release or the self timer is where you shouldn't use it, as there's no movement at all to be detected, so it's really a waste of time, and might actually damage the iS system.
