The advantage would be that not all regions make you silly rich.

Also its to me a question of game atmosphere. It just doesnt feel right that some desert oase has the same size as Rome. I would like to see the potential to be only in the cities which historically developed to metropoles:
Rome, Carthage, Anthioce, Alexandria, Byzantium, Syracuse, perhaps Athens, although it in our time frame really couldnt compete with those other cities in terms of inhabitants.