Heathrow Director: your last sentence is perfectly reasonable, of course. But there might be a time when the Commander HAS a good reason, and that is when he or she really needs to feel that they can and should make the final decision. Pretty rare these days, I admit.
However, only yesterday I saw and heard a controller giving take off clearance to an aircraft when there was another aircraft backtracking the runway. The pilot of the first aircraft didn't take-off! ATCOs, like anyone else, can err in judgement: no one is perfect all the time.