Consider the following hypothetical identities: lazerboy on Kuroshin and Advogato, lazerphazer on Slashdot, email@example.com, and JohnDoe on Meatball. Each of these (hypothetically) identifies the same referent. Consequently, if the meaning of a name is contained in its reference, is it meaningful for a community like MeatBall to insist on the use of real names?
If you wear a business suit to your job interview, you will project a substantially different image than if you wear torn-off jeans and a flannel shirt. Likewise, lazerboy and JohnDoe might denote the same person, yet they connote a different set of attitudes and intentions. Since any community -- especially an OnlineCommunity -- is a limited-knowledge environment, the choice of identity is a significant conveyor of ParaLanguage, particularly in initial interactions before one has formed a more complete impression of an individual. As long as there are negative reasons to choose pseudonyms (see BasisForPseudonymity, VulnerabilityToCommunity), the use of pseudonyms will somewhat imply those negative reasons.
However, discriminating on the basis of how we "dress" our names is old-fashioned. OnlineCommunity is a completely new context; why can't we just choose to do away with such old-fashioned notions? Moreover, in today's international context, who's to judge what's a real name, anyway?
However, the importance of appearance is a long-standing argument that will probably never die. Not all people agree that such an emphasis is necessarily entirely "old-fashioned".
See also: DressCode, IdentityValidation, VulnerabilityToCommunity, EnforceResponsibility, PenName, BasisForPseudonymity, FocusOnIdentity.