I agree that the deposit is to prove that the client is psychologically committed. There are times when I am pre-booked for months and to have somebody back out at the last minute isn't cool. Before I started taking a deposit, I had somebody do that to me. I had already lined up a crew and bought the supplies. That was when I started taking the deposit and it hasn't happened since. The deposit is to hold the date and also once you have purchased specific colors and products, it is only fair that the client go through with the plan.