Your premise is wrong. You assume that the entrance essay is "graded". It's not. It's meant to give the admissions board more information about your character as a student, and insight into how you express yourself in the day-to-day.
While the essay isn't graded, it is *judged*. It is judged based upon your ability to convey your thoughts and ideas (along with your test scores, 2ndary school grades, extra-curricular activities, etc), and whether that type of expression fits into MIT society.