Although "content" (ideas and clarity/strength of argument) always weighs more than "mechanics" (proper grammar and punctuation), when people get to the "real world," they are definitely judged by their ability to write "properly." No teacher would mock or belittle a student's ideas for using colloquial or "black English" (although they will correct it, a little at a time). But a prospective employer will, the person reviewing complaints at any organization or agency will, and sometimes the posters on messageboards will. If you can't use commas correctly or spell or work out subject/verb agreement, what else don't you know?
Especially in a "writing center," this is not a helpful policy, at least on its face.
We all know how to speak informally, sloppily, with the accent and phrases of our region. If we want to write that way to set a certain tone, fine. Being able to write properly, though, is like knowing the difference between how to behave in church as opposed to how to behave at the local bar.
