Agree with you that short written test before interview is a good option for testing the written communication skills of the candidates. Same as oral communication that can be tested during interview, I do think that effective written skill is so important to nowaday IT staff. I say "effective" but not "good" because I don't focus it on beautiful wordings and grammatical error-free, but do focus on how the message can be conveyed in a sensible, logical, precise and concise way.

What I saw over past ten years in my working environment is that it is the world of email in which most of the communication among staff, including IT guys and users, are done through email. I've seen countless cases that so much time was waste for clarification in communication just because of poor written message delivered by the staff incompetent in written skills (but he may have obtained MCSE, MCSD, OCP, CNE, ABC, XYZ, superman certs....)

The IT job indeed requires effective oral and written communication skills and so the ads say. What it seems to make the ads meaningless just because of the lousy IT manager who ignores the chance for the screening the candidates during the interview. Anyway, the manager would suffer it afterwards.