the technical side is that, the more times you start the apu, the quicker the starter motor will wear. Which is indicated by a wear pin which is part of our weekly check {A319}. We have had a letter from the airline we work for saying that they lost x amount of money through having the apu running for no reason. {I think it was more directed to us engineers}. Another thing to remember is that for some reason during the turn-around, you go to start the apu and it does not start and you have no technical personal available, delays creep in.