"Pay no attention to the criticism of men who have never themselves written a notable work."
Ezra Pound, "A Retrospect"
Last time I suggested that teachers of writing bring authenticity to a lesson when they write on their own. Being engaged in the work of writing gives those who teach experience navigating the difficult relationships that exist among writers, their subjects, and their audience. That rhetorical triangle is basic whenever we sit with pen and paper (or keyboard and screen -- in any size or shape). Anyone listening to advice from a writing instructor has a right to expect that that instructor knows something of the writing process. Writing instruction is very different from interpreting literature. At times reading and writing are related and work together, but the act of writing introduces and requires different strategies and expectations. We can summarize a plot and talk of symbols and metaphors based on our experiences reading and our (often learned and, therefore, artificially developed) ability to interpret texts. That is very different from sizing up an audience or trying to create a full argument that will somehow be accepted by a variety of readers.
So. I am arguing that teachers of reading bring a very different skill set to their work. And it's not reasonable to believe (or to suggest by how we hire and promote individuals in the academy) that interpreters of literature can teach writing or, for that matter, that a content specialist is able to teach with nuance and sensitivity the requirements of writing to a peer or general audience). Unless, of course, they write. Therein lies a complicated (and sometimes too simplistic) argument for the link between scholarship (here meaning writing for an audience of one's peers) and teaching.
One part of this argument is that writing forces a "scholar" to fine tune an argument and arrange clear and useful discussions into a full essay. That pushes an individual to understand content and then translate that content for a wider audience. Unless you are trying to create something to be read by peers (or by those who know more and better things than you do) it is likely that you are not sufficiently challenged. If you are writing to an audience of novices, it's easier to get away with less than stellar thinking and less than complete control over material (I understand the snottiness in that remark: a specialist who writes for a general audience faces a whole other set of demands).
The point to writing within a discipline is to comes to terms with what you know and to bring that knowledge to readers with a hope that your work will help create a different understanding of a topic or idea. It's not really about putting yourself on display. Nor is it about writing to demonstrate that you have some facility with the topic. That should end once you complete courses and requirements.
On a deeper level the struggle to put ideas into writing (or the demonstration of a struggle with ideas, which really is a good deal more interesting) is what ties us to our students. We should bring our experiences as writers to them to show how writing works -- how it works both to demonstrate knowledge and, perhaps more importantly, how to create new knowledge by writing our way to see it. If we don't struggle with all of that, how can we in good conscience stand in front of them and instruct them on the fine points of writing. If the last thing I wrote was 5 or 10 or 20 years ago, I have no business teaching writing. Further, if I haven't really challenged myself by submitting my writing to an editor or editors or to a reading public (which is the only way to demonstrate that you are serious about writing and being part of a discipline's conversation), I have little authority to judge my students' work.