Many years ago I had a Tristar down in Mexico with a flight back to the States. It was Easter Monday with a failed TCAS. So I spoke to an FAA official to see what could be done. After a brief disscusion he said to me "I am going to ask you one question to which you can only answer with a yes or a no". The said question "do you have TCAS fitted". The answer was of course "Yes". The said official then said that there was a percentage of the civil fleet that flew in the states in accordance with the relevant MEL but safety was increased as there was a larger number of the fleet with TCAS operating. Mind you that was back in the 90's