This doesn't apply to a lot of us, but some.
Your education is for you. Don't (always) suck up to your instructor.
I had to analyze a poem a few months ago for a class. I dislike poetry in general, but this one was downright bad (IMO) and I made my opinion known in what I wrote. Not only did I end up with a 100% on the assignment, my instructor thanked me for my honesty and asked if she could use my essay as an example for future classes for both my analytical approach and my honesty about the poem.
Some instructors, yes, will totally let personal bias affect grading. You can usually see them coming from a mile away. But if you are told to write based on your view of something, then damn well do so. The education is for you, not for them.
I had to write a 'professional-quality' review of a movie this week for class. I thought the movie was garbage. You better bet your ass I made it known. So what if my classmates wrote glowing reviews. Maybe they thought it was great. Maybe they're trying to suck up to the instructor. I thought it was garbage. In the end, this education is for me and me alone and I thought Insidious was utter drek as far as horror films are concerned.