One of the distinguishing characteristics of the Great Recession is how it has impacted the mix of restaurants with servers and without. The evidence is pretty impressive that customers want high quality food that they have to essentially get themselves as opposed to having a server bring it to the table.
Does that say that customers prefer no service, of course not. The implication is that the servers are not providing any value to the dining experience and that is why customers do not want to pay for it. Servers are not entitled to 15-20% because they are there physically there, rather they need to add value. They have not and if they choose to continue the process they will succeed in obsoleting themselves.