[Ilmesnkie_Jones]

Even if their perfect its not really good enough outside of say standardized testing because they can't integrate what they learn about the individual students from grading their papers and then use it tutor/advise/grade the student.

A holistic understanding of the individual student and how all students are doing in a course is important. It allows the professor to tailor the course or see where weaknesses in understanding are.

Also I would imagine that there are some exceptional essays which because of their unique nature would slip by the grading system. Subjectivity is a plus and a minus of having human markers.