There's no oppressed person in a police state signing up for Facebook that reads a EULA and concludes "well, Facebook said I have to reveal my true identity, I guess that's the way it is."
This is a fairly significant and under discussed problem. Rush Limbaugh is probably safe from harassment... that dude commenting in some political group is not so immune. This creates an interesting chilling effect.
This kind of policy goes a long way towards the perception of professionalism at any web site. You're probably not going to take a person named "Spazmo Jones" as seriously as you would "Amy Wong". It's a sort of broken window theory.
(And yes, all of the examples above are pen names people actually tried to sign up with!)
Does it, though? Looking at HN usernames, some are clearly names (mine, yours), and many are probably not (patio11, JupeterMoon, hackuser, on__3, to use some from the comments of a neighboring thread). I don't think this detracts at all from the professionalism of the discourse here, nor from the perception thereof.
Similarly, I wouldn't weigh the professionalism of a hypothetical StackExchange user differently if their usernames were 'SparkleMermaid', 'ArmadilloRocketeer', or 'Iheartpython' -- I notice the content of their writing before I notice their name. Heck, the sandwich-themed name of your company shouldn't make one pre-judge the communities you've built, or the perception the work you've done to build them.
Am I that much in the minority that I don't care if your username is your initials + favorite 3 digit number, your real name, or a reference to your favorite Star Trek episide? Is this because I grew up in a time when handles were either assigned-by-name or completely made-up, depending on what system you had joined? Maybe it's the influence of having been a lurker on Slashdot and HN, where clever nicknames were commonplace, yet one's choice of nickname nearly never was the basis for judging value of contributions.