This is why I'm skeptical of the suggestion to prefer a natural primary key like a username. It works fine... until they day the business asks for changeable usernames because BigClient is now LargeClient and can't stand anything to still have their old brand identity.
Even in the natural key's playground (in my mind) of data warehousing, you still have to manage changing dimensions, and the inevitable reality that your fact table was actually another dimension table all long.
