[GeeTwo]

Actually, a kiwi drive (as with any holonomic drive) should already be able to rotate about any desired center of rotation, without the legs. To rotate around one of the wheels, just keep that wheel fixed, and rotate the other two at the same speed and direction (clockwise or counterclockwise). if the rotation center is desired to be closer to the center of the robot, rotate the pivot wheel in the same direction (but more slowly). If the rotation center is desired to be farther from the center, rotate the pivot wheel in reverse direction. I'll look around later to see if anyone has done the kinematics in terms of center of rotation and rotation speed; usually they're presented in terms of translation and rotation.

Unless the leg did more than just sit there or go vertically, they would presumably only be useful to stay in place. There are certainly times and games for which this is useful - planting to take a shot, for example. For defense (apart from being an obstruction), they're not likely to be effective except possibly in a few oddball orientations.