Servers make less than $2.50/hour. They depend on tips for a living. The government even taxes them based on expected tips.
If you cannot afford to tip, don't eat out. Otherwise, everyone should tip at least 15% unless the service is bad.
It is no longer the case that tipping is for extraordinary service. It is the only for way servers to make a living wage.
I don't mean to sound uncaring because I do care but I'm not responsible for what their employer pays them and that the government has exempted them. Servers are not my employees. Maybe, owners should increase the price of the food and give an at least minimum wage. If fast food is required to pay minimal wage then restaurants should too. Tips should be well deserved and extra.
Call me heartless but the servers need to be paid by the restaurant owners. I still give 15% post tax total on average, but it has to be good service. Slow gets 10%; piss me off and they get a dollar.