Any half busy airport where the controllers waited for the entire aircraft to be over the CAT 1 holding point would suddenly be a lot less busy! As has been said as long as the aircraft has vacated the physical runway itself then next aircraft movement can take place. That said I've seen some amusing calls from pilots calling vacated, the pilots may be, but the other 300pax are most definitly not! Especially important in LVP's, where you are not considered vacated until the aircraft has passed the last yellow lead off light. (UK)