Manish over at Damn Foreigner relates an exchange that highlights the important difference between anonymous and pseudonymous writing.
Apparently Andrew Sullivan was criticizing Atrios on Minnesota Public Radio for not revealing his "true" identity, claiming that no one can evaluate his positions without knowing who he is. However, Mr. Sullivan misses the fact that we can evaluate Atrios' writing in its own context because it is published under a consistent name; Atrios has chosen to allow his writing to stand on its own merit, a decision Mr. Sullivan should appreciate as a fellow writer who has written pseudonymously himself (follow that link at your own risk). Pseudonymous writing has a long and noble history, including works such as The Federalist Papers and authors such as Mark Twain/Samuel Clemens. As long as pseudonyms are used consistently, writings done under pen names are as easy to criticize and evaluate as those done under real names. (And what's the difference between a real name and a consistently-assumed false name, anyway?)
In contrast anonymous writing is unattributable and often irrefutable because an anonymous author can change positions and arguments at any time without having to maintain intellectual honesty or consistency. Anonymous writing has as long a history as pseudonymous writing -- particularly in extremely oppressive conditions -- but it's generally rightly seen as cowardly and prima facie unpersuasive unless the work can stand entirely on its own without any external support (as could be provided by other works from the same pseudonymous writer). Anonymous writing can often serve to call attention to a cause, but the actual work of building a case must afterwards be accomplished by writers with names.
Eponymous, anonymous, and pseudonymous writing all have their place, and freedom of expression demands that all be allowed; ultimately the decision belongs to each individual author. There are trade-offs for each.
As for myself, I use my real name because I'm not particularly afraid of persecution and I enjoy seeing my name pop up in Google.