For some people (not me), this is an issue of safety; I somewhat agree with you that people who genuinely need to protect their identity from The Internets might be better off not using social networking. There are some corner-cases, like famous people who want to be online to communicate with their fans, and with their families, but don't want their families to have to deal with unwanted publicity, but the number of people in the world who are that famous is very small.

For me, the bigger issue is correctness: through various quirks of history, different groups of people know me by different, unrelated names, and I'd like my friends to be able to recognise me as we encounter each other in this new medium. Pseudonymity is a different thing to anonymity.