You do not have to look far as to why one might want to remain anonymous. I submit The Federalist Papers[0] as exhibit A. If the Founding Fathers deemed it necessary to publish under a pseudonym then who are the likes of Facebook and Google to say otherwise? Yes, you may say that FB and Google are private entities and as such we must play by their rules. But I would retort that FB and Google are the default market places of the day for connecting with people and spreading ones ideas. I submit they have an obligation to maintain anonymity for those that choose to remain anonymous. The Federalist Papers were not self published. They were published by The Independent Journal[1], a journal of the day. That journal saw fit to publish those works anonymously under a pseudonym.
Anonymous publication must remain a viable avenue in the digital future if we are to maintain our character as a free nation. There can be no two ways about it.
[0]http://en.wikipedia.org/wiki/Federalist_Papers [1]http://en.wikipedia.org/wiki/The_Independent_Journal