I can only assume that if they want immobilization, then they REALLY want immobilization; you're right, going over the ankle seems overkill, but if you're allowed to rotate your ankle it may translate up to the knee and pull the quad(?); strange thing is that you can still put strain through the quad while in cast by doing an isometric contraction, so i don't really see the upside of full casting vs brace and stern warnings to rest it. Meanwhile, continue to exercise arms and other leg, if allowed by doc, to maintain as much strength as possible. I wish you a speedy recovery.