Let's start with admitting a portion of the case for rubrics: rubrics seem to allow students a certain degree of certainty and predictability. They act as a signal to students, a sign (or reminder) of what the teacher is "looking for" in a good quality bit of writing. Advocates say that rubrics are supposed to signal to the students "what is required" and in so doing "take the mystery" out of grading.
The problem is that they don't. Not really.
Here's an example of what might go into, say, a high school writing rubric:
CLEAR, WELL ORGANIZED, WELL DEVELOPED IDEAS: 25 points* Main idea (thesis) is clear.
* Each paragraph has a clear, effective topic and concluding
* Supporting details clearly relate to topic sentences in a meaningful,
* Content in body paragraphs is appropriate.
* Transitions are used effectively.
* Introduction, body, conclusion provide logical sequencing of ideas,
leading to understandable, appropriate content that is free of
superfluous information. Attention to audience is evident.
So we know from our rubric that the student has been told that each paragraph should have a "clear, effective topic". But notice: nothing in the rubric actually helps the student know how to write a paragraph with a clear, effective topic. It merely tells the student that this is what they should do. And herein lies the problem. Let's call this the Lopez Principle:
If a student actually has learned and thus knows how to write a paragraph with a clear and effective topic, the student will already (simply by virtue of understanding that skill) understand that this is what one should do when writing paragraphs.
Putting this requirement in the rubric doesn't actually take any mystery out of grading at all, because to the extent that there is any mystery in the grading process -- to the degree that the student genuinely doesn't understand what the teacher is "looking for" -- the rubric merely gives that mystery a name. It doesn't explain anything.
But if a teacher has taught how to write a paragraph with a "clear and effective topic", at least if it has been taught properly, then the students already know that this is what they are supposed to do. The teaching is in the teaching, not in the rubric. Put another way, rubrics shouldn't be shorthand substitutes for textbooks or good articles about writing. They're not really suited to the task.
So the value of the rubric can't really be educative, its value must be as an assessment tool.
Now, perhaps there is some value in having a predictable, established set of grading criteria. I'm willing to entertain the idea that it's a good thing, for instance, to let students know that they'll lose a point for each spelling error, up to a maximum of 10 points. That might seem "fair" in a way that simply giving an otherwise A paper a B+ and writing "too many spelling errors" doesn't.
But this sort of discussion is all about seeming fair, not being fair. A teacher who uses an undisclosed rubric that the students never see is still "being fair" in the sense that all papers are being treated equally and according to a uniform standard. People who argue that grading of papers is arbitrary have never graded any significant number of papers; grading is very rarely arbitrary. It might be mysterious to students who haven't been taught about quality, and there may be elements of subjectivity to it, but it's not arbitrary.
What I suspect is that rubrics are really used as a way to avoid conflict over grades (both with parents and students), to give teachers something to point to and say "Bobby didn't set forth a clear and interesting thesis, and it's obvious that he was on notice that this was required because it's right here in the rubric." Never mind that the reason that Bobby didn't set forth a clear and interesting thesis is that he doesn't actually know how to. That you, the teacher, failed to teach the principle is irrelevant; you put it in the rubric, so the student is responsible!
That gets the moral principle backwards, though. The student is responsible not because it's in the rubric, but because you taught him how to write a paragraph with a clear and effective topic. (Again, unless you're using the rubric as a textbook, a task for which it seems ill-suited.)
What's more, I think that rubrics pose a danger: students can come to see the writing of a paper as a sort of checklist of things to accomplish, rather than an organic process that involves first the birth and development of ideas, and then the translation of those ideas into writing, and finally the hard work of editing. The rubric expressly sets forth the paper as a grade to be gotten, and focuses student attention even more on the grade than it is already, rather than trying to shift some of that (entirely natural) grade-focus onto the act, the process, of writing itself.
On top of that, many pre-generated rubrics are, insofar as they are normative statements asserting that one should engage in a certain type of writing, utterly false. I've seen a lot of rubrics that promote some very, very bad writing.
Rubrics can be convenient for teachers, mind you. It's easier to put a frowny-face mark next to "Clear and effective topic" than it is to explain in notes to your students' papers what about their topic is either unclear or ineffective. But teaching writing is hard work, and the rubric shouldn't be used as a substitute for that work.
So... I'm not a big fan of rubrics. I can see their use, maybe, as a sort of "before the fact" syllabus in a series of writing lessons. But the sorts of writing that one engages in outside of school just doesn't have rubrics. Students need to learn to write for a purpose, not to a rubric.