We are going to need to know everyone's last names so we can find each other more easily in XIV when it releases, but I don't see it as an important enough thing to force into everyone's usernames.
So instead I'm going to make it a profile option (and once I figure it out, make it appear on the left in your posts in place of stupid things like "location", I mean really... who cares about that given what we're doing? lol) I will allow the option of having your last initial in your username or leaving it at just your first name. That way the forums won't be cluttered with lengthy names all over the place.
I don't expect there to be a lot of similar first names but if there are then the initial will be required, if there's a double of even that then I'll put in the full last names of both persons (or make their last names their usernames, just to mess with people's heads! >:D )
If you don't know yet what you want for a last name there's no rush, but do hurry! :p (yes that contradiction was intentional)