When you reach 999/1000 Exalted with one of the four home cities, I believe you are supposed to receive reputation with the other three cities when you should get rep with your exalted one.
For example, if a quest yields 100 Orgrimmar rep, but I'm 999/1000 Exalted with Org, then it should give 33 rep to Undercity, Darkspear Trolls, and Thunder Bluff.
Currently, it does not.