What is the advantage of IS when the lens is already mounted on a tripod?
Super-telephoto lenses are especially prone to vibration. In use, they are quite cantilevered, and the majority of the lens’s weight is toward the front end. They have so much surface area that they catch breezes like a sail. And every bit of vibration of a 600 mm lens is magnified 12 times–24 times, if you are using a 2x teleconverter. Super-telephoto lenses quite often vibrate regardless of good camera support, and photographic sharpness with super-telephoto lenses still benefits significantly from IS, even when the camera and tripod are sturdily supported.
