The beautiful thing about writing (I say as a writer and as a writing teacher) is that it precisely is NOT objective, and of course we teachers are judging subjectively (so are employers, consumers, and voters, by the way).
The only way a program can grade is by comparing a sample of writing against a very narrow standard. The essays students are asked to write on standardized tests are artificial topics and the testing situation is also artificial (time-limited, w/o access to resources, revision, etc). Standardized writing is an oxymoron.
Update: I also like how this commenter points out the humanness of writing:
Creativity is human. Critical thinking is human. Empathy is human. Moral dilemma is human. And above all — meaning is human.
Technology should exist to empower, and strengthen, human relationship and meaning. Never should we allow it to replace it.