I can trivially judge the necessity of service but unless I am an expert in the domain I cannot judge the quality of service and very often the entire point of the service is to outsource expertise.
I sent my car in for an oil change. They didn't replace the clogged oil filter and it started leaking a little. I had to sent it in again.
Clearly I know that I need an oil change but unless I do my own inspections (the very thing I am paying for) how am I supposed to know that something went wrong?
The entire point of this service is to improve your subjective experience - if you can't evaluate that then you probably need a legal guardian.
The waiter is the face of the establishment, but there are many things out of the control of even the best server.
Which gives them the incentive to kick things off in the kitchen, and good places share tips with kitchen staff for that reason.
It's their job to make the experience pleasant for me - I don't care how - tip reflects how good of a job they did.
If I don't like a place I let the owner know and reduce my spending. If I like the place I tell the owner and spend more.
Tautology anyone?
That's ridiculous. If you're not qualified to judge the value of service to yourself then your probably not qualified for a lot of spending decisions in your life.
Reasonable expectation ? If you're a reasonable person you'll have reasonable expectation and it's really easy to notice when someone makes the experience more pleasant.
And if a car mechanic returned your car smelling like shit would you not be mad because he had digestion issues ?