If we've learned anything about reading and writing in the past 20 years, it is that each person brings to the activities such individual perspectives that it's amazing readers can agree on any aspect of the meaning or value of texts at all. Heraclitus, commenting on the vagaries of life in Greece in the 6th and 5th centuries B.C.E., said you can't put your foot into the same river twice, since it is always changing. Some wag amended this to claim you can't even put your foot into the same river once. Like it or not, students must learn that their human readers will be inconsistent, a hard and frustrating lesson. Writers learn to take their best shot, but they all know it is impossible to predict a reader's response with complete accuracy.
Professional readers are no more consistent or predictable than any others.
As a manuscript reader, I know I've accepted essays for publication that others have rejected, while ones I've turned down have found their way into print, where they have been well received. Writers and editors learn that this is part of the natural give and take of the word game. It is harder, and often more frustrating, for teachers to face up to the fact that they too disagree over the relative value of different texts, whether they are student-generated essays or lists of great books produced by the cultural elite. That realization may not change how teachers grade, but acknowledging it would help them teach their students to expect real, rather than ideal, readings of their work. Teachers want to grade less, but I doubt many will want computers to do their reading for them.