When I snapped my 16-35 f2.8 in two I was told by a tech at CPS that the plastic cage that the metal mount screwed to was a designed breaking point. He said that if camera and lens were dropped it was supposed to break away before too much impact was transferred to the glass.
My 16-35 was mounted to a 1VHS and fell onto a concrete floor. The four screws snapped tabs off the plastic cage, the mount stayed on the camera, the lens rolled away with no damage to the glass, barrel, or filter threads, all the ribbon cables snapped and it cost $150 to fix. They replaced the plastic cage (they actually offered to sell me one so I could repair it myself as I was in a fairly remote spot), and the ribbon cables, no damage to the glass.
I am very happy with the design, I think it worked as intended and prevented my lens being damaged more severely.