If you’re being asked to write—anything—for free, I just hope the person asking is your mother or your kid (or the person who shares your bed). Because if the asker doesn’t fit into one of these categories, he or she is no different from someone in a dental chair saying to the dentist: “Oh, and I just want to confirm that you’re not going to charge for this crown, right? I’ll show it to everyone; it’ll give you great exposure.”
Tim Kreider has published an exceptional essay in the New York Times that tells why it’s wrong to write for nothing: Slaves of the Internet, Unite!
You owe it to yourself (and to every writer who ever hopes to make a living writing) to read this article.