This is a response to some frustrations which a student expressed to me, and which are, I think, typical of a lot of people who are introduced to the subject. If you've taught philosophy for any length of time, you know where this student is coming from.
I know that philosophy, by its nature, can be frustrating, and it requires somewhat different skills than what you might be accustomed to using in other classes. I make no apologies for that; the discipline of philosophy is what it is.
There is a common conception when students come to philosophy classes that everything falls into two general categories, fact and opinion. If it is a matter of fact, we can settle it by some broadly scientific method. If it is a matter of opinion, then different people have different opinions, and we are all entitled to our opinions. Philosophical questions are all matters of opinion, and therefore there is something absurd and perhaps even offensive about grading a philosophy paper.
I think this neat division of everything into two boxes, fact and opinion, which we learned all the way back to fourth grade at least, is a distortion of the truth. Just because we cannot settle a question to everyone's satisfaction through a well-defined method doesn't mean that there can't be better or worse reasons for believing what we do, or that we shouldn't be aware of the reasons for and against what we believe. Whether it is worthwhile to spend time working through one's world-view and putting a lot of reflection into that, or whether there are other, more adequate uses for a person's time is not something I can answer for someone else.
But people do have decisions to make that affect their lives. They have to decide whether to become actively involved in one of the world's major religions, and for Western religions, this invariably involves belief in the existence of God. They have to decide what they think is real. They have to decide what sources of knowledge they can rely on, and what sources they should call into question. They have to decide by what rules they decide what is right and wrong. And they have to decide whether they really think they have a free will, and also whether they are the persons who have an eternity to look forward to, or whether it all ends with the grave.
Even if you have decided all these questions in your own mind, others around you are making those decisions, and I take it you are in conversation with them.
As for grading philosophy papers, I do not grade papers in philosophy on the basis of whether I agree with the person's beliefs. Two things I look for are 1) How clearly you state your own position, and 2) The extent to which you carefully reflect on and articulate why you believe what you do as opposed to what others believe.
I wouldn't have ended up in the job that I have if I didn't think that philosophy was a valuable enterprise. I cannot make that judgment for other people. However, since we're in a philosophy class, we have to do the philosophy curriculum. After 23 years of teaching experience, I can tell you that you are not alone in your philosophy frustrations.