What is the purpose of having grades?
A professor who I used to work with once gave me an explanation that I found quite useful for understanding the purpose and philosophy of grading.  Universities, he said, must always struggle with a tension between two educational goals, illumination and certification.


Illumination means the intellectual development of the student, bringing them a deeper understanding of a subject, its relation to the world, and the deeper issues it may touch on.  From the perspective of illumination, marks are intended to be feedback to students that helps them realize weaknesses in their understanding so that they can fix them.
Certification means evaluating a set of skills acquired by the students against an objective standard, to attempt to measure their fitness for certain tasks or professions.  From the perspective of certification, marks are intended to be objective judgement of the fitness of the student for carrying out tasks requiring the skills taught in a class.


These two are often in tension with one another because certification pushes teaching toward rote practice and standardized testing and grading, while illumination pushes toward more open-ended exploration and interactive formats which can deliver much more benefit for apt students but are often very subjective.  Most classes try to deliver both, to varying degrees of success, though some classes may almost entirely hew to a single side of the balance.

You need to decide what you're after from the classes that you take.  From your "pay for service" tone, it sounds like you want the career value that comes from certification.  But certification isn't valuable if the standard can be easily negotiated, and so professors have to set a standard and stick by it.  Sometimes they are even forced to by regulations.  From the perspective of certification, trying to negotiate for a better grade is trying to cheat the system and reduce the value of everybody's grades.  

Moreover, one of the "meta-skills" that is always being certified is the ability to figure out what somebody wants from you.  If you're out in industry and you deliver the wrong thing because you misinterpreted your client's needs and didn't ask for clarification, it will be difficult to argue that you should be given more partial credit.

If, on the other hand, you're after illumination, then grades are less important to begin with.  In that case, it's more important to understand why you got the grade that you got, so that you can improve your understanding of the material.  If you want illumination and you aren't getting it, you need to switch courses, majors, or institutions.