Total madness. If your operating your runway at or close to capacity you cannot do without conditional clearances, especially on single runway ops. A busy AIR controller, planning, coordinating with other controllers and on phone lines does not have the luxury of the time to start issuing the line up/crossing clearance at the exact right time as the landing/departing aircraft has passed. A few seconds to late will be the difference between a gap working, or a go around. And as mentioned, pilots will be wondering whats going on. A great idea for quiet airports or for looking good in a safety meeting, but ridiculous in real operations.