The argument for political dissidents in nonfree countries is valid but old (in many cases the regimes will catch the dissidents regardless). The value of pseudo-identity is not just anonymity, but the ability to have multiple identities. We shouldn't be forced to use one identity for the various areas of the internet, just like the comments and actions i make on HN are totally unrelated to my facebook community or my twitter community. Single-identity is a vehicle for tracking people, and i can only think of evil reasons to track people across communities. Even in real life, we dont carry our real identities around, you don't tell your full name to the cashier in the supermarket or to people in the street. If you get into a fight, you don't start by exchanging phone numbers.
In fact, offline real identity is less dangerous: unless you kill their mother, people you don't interact with will forget you in a matter of months. The internet is permanent.
But why am i wasting my time, facebook people think like infants anyway.