The body-side gasket is limited to the EOS-1V, the 1D-series bodies, and the Extenders II and III. Most recent L-series lenses have the lens-side gasket, as do the Extenders II and III. The lens-side gasket alone probably improves the seal between the body and the lens, but presumably not as effectively as when there is also a body-side gasket. If that is so, it seems a bit cheeseparing of Canon not to provide a body-side gasket on non-1D prosumer bodies such as the 5DII and 5DII and 7D, which otherwise have quite god weathersealing. It is not exactly a piece of high technology.