To be honest, i've always thought using usernames is a security risk as other users will know the login id of the person. Using real names is better since CB, J! and SMF all allows users to change their real names. Allowing users to change usernames in CB will result to several problems, most important is that many J! extensions use usernames as an SQL identifier. When a user changes his username and another user uses the previous user's original username, it can cause confusion in some extensions.

Not sure if you get what i mean