If you run a restaurant, you don't care what the psychological make-up of your customers are, as long as they have cash/plastic in their pockets. What you do care about is an effective means of extracting that money from them. And a waitress/waiter is the most effective means of extracting that money.
Actually I've ran a restaurant before and my primary concern was whether or not the customers were happy with the food and the service not the firmness of the waitresses arse. But I'm silly like that.
Ohhh and good reliable wait staff is more preferable than some super model wannabe primadonna.