If the argument is that servers will only provide good service if they are given tips as incentices, then why should this only apply to servers?
Does it mean that customer service representatives should be tipped to encourage them to really try to help you?
Or that you should tip school teachers, dentists, or any other employees that happen to be facing customers?
For all these other professions, if the employee doesn't do his job well, he will eventually be replaced by someone better - why should this be any different for servers?