I struggle to imagine putting together a solid argument for why we wouldn't want all of our students to be capable writers when they graduate. Writing well is an obvious good. While much fuss was made about newfangled twenty-first-century skills, one very old skill that seems to be only increasing in importance is writing. Here we have the importance from an economic angle:
Unless you're a future miner (apparently), writing matters — especially if you want access to the salaried jobs that typically coincide with a middle class lifestyle.
And of course, just as school should not be conceived of as a place that produces widgets prepared to enter the economy, writing is not only about dollars and cents. A proficient writer is able to journal through mental health struggles, articulate complex thoughts, and communicate in a measured manner. Writing makes us better readers, better thinkers, better speakers, and better listeners. Through writing, we can inform, explain, argue, entertain, and inspire. This isn't just for teacher/blogger/nerdy types (I don't know any of those, of course); it's for mothers, employees, citizens, dreamers, and spouses.
Unfortunately, those who graduate today with the ability to write have one more huge advantage: they're rare. According to the most recent (as of this writing) Nation's Report Card on Writing, only about a quarter of kids and graduates are able to write proficiently.
We're not talking about the ability to mimic Shakespeare here — proficient writing is the ability to compose an email that someone can read and understand without difficulty. As NAEP says in their report, this is the ability to accomplish one's communicative purpose in writing. When you're proficient, your writing does what it was supposed to do. More than a quarter of our kids need to be able to do this.
The solution? I think that David Conley says it best:
“If we could institute only one change to make students more college ready, it should be to increase the amount and quality of writing students are expected to produce.” (from “The Challenge of College Readiness” in Ed Leadership, 2007)
To paraphrase Dr. Conley (who, for what it's worth, was researching college and career readiness years and years prior to the Common Core popularizing the term), this is what it takes to make the next report card a better one:
Step One: Increase the quantity of writing students are expected to produce
Step Two: Increase the quality of writing students are expected to produce.
In most schools, the fruit hangs so low in these strategic initiatives that it is practically underground.