Dennis,
To many instructors, the simple act of conveying a simple idea in written form using correct grammar would seem to an assumed ability for college or professional trade school students. My guess would that that is more often not the case than is the case. In a writing class, you have the perfect nsetting for "pretesting", or perhaps preassessment. Whatever we call it, determining which students need more help early on provides the best opportunity for the instructor to work with these students to help assure they meet the minimal competencies in the course.
Barry Westling