Don't forget that when you are in the hold (with a lot of other aircraft usually!) that your modes a and c could be unreadable and therefore we rely on accurate reports from pilots in order to descend in the hold.
As an adjoiner to this sometimes we rely on a runway vacated report for one reason or another (ie: can't see runway, surface radar broken etc) in order to clear an aircraft to land or depart, you're vacated when your tail is passed the cat1 hold not when you've turned the nose wheel off the centre line!