I forgot to say something important about disclosure of identity.
Schwartz and Ward’s list of identity-contributing attributes, which, according to my hypothesis, is also the list of details that opaque bidents tend to keep hidden from others in Second Life, also correspond to those characterstics on which people have historically been segregated. It makes sense, yes, that those attributes that outline who we are, those attributes that give us our identity, also serve to establish the “in” and “out” of elites and cliques. Look at the list again: it’s easy to find recent past (and current) cases of discrimination based on race, ethnicity, nationality, first language, gender, religion, social/economic status and sexual, cultural and political preferences.
What is this to do with SL? Well, in addition to the already mentioned reason that bidents try to avoid disclosing information that would cause their two identities to clash with each other, I now hypothesise that an additional reason for bidents to stay opaque is that they try to avoid being prejudged; prejudice is easy to minimise if we hold those details that are often used as a basis for discrimination. Note that this applies to any kind of opaques, not to bidents only.
In summary: by being opaque, SL residents minimise the risk of being the target of prejudice from other residents, thus avoiding potentially unfair discrimination.