I've always thought we tip in this country because people in service industries are inadequately paid — and minimum wage is not adequate pay, especially if you're a single parent, as many in low-income jobs are.

Tipping is a form of charity. If you're affluent enough to eat in restaurants, you're affluent enough to help support the poor. If you don't want to do that, don't go out to eat.

In my state, BTW, an estimated tip income is calculated as part of the wage, so restaurants that are not part of interstate chains but are just local operations do not have to pay servers minimum wage.

I'm afraid the plumber example isn't really appropriate. You don't pay a plumber based on how well you think he does his job, just as you don't pay your restaurant bill based on how good you think the food was. You ordered something, thereby agreeing to pay a certain fee. It'd be like saying that you didn't like the doorman at your apartment complex and so you were adjusting your rent to reflect that. You can choose not to tip the doorman, but you can't choose how much of your rent you'll pay.

That said, I do think the tipping culture has gotten a little out of hand. I'm more than happy to tip a masseuse (on the rare occasions I get a massage), a delivery driver, a hairdresser, a server or a bartender. (If I drank coffee, I would include, to a certain extent, baristas.) Otherwise, you really don't need to throw extra money around.

In my state and several others wait staff does get full minimum wage.