They don’t, I mean registering your username/basename is not a requirement, they chose the registration as the default to make it easier to be found. But you can get away with not registering your username/basename and instead exchange with your contacts you ID number, and with that besides able to choose whatever username/basename, there’s no central directory to find you, which is good depending on your use case, but the Jami guys are right to say that makes it virtually impossible for others to find you and establish a conversation unless you exchanged somehow your ID numbers, but that’s not actually finding, :)

That option is a one time choosing, when creating the account though.