Nobody that I can recall as being in on the ground floor of this project made "efficiency" a consideration. It's supposed to be about creating 3x the positions, 3x the cultures, and 3x the fun. Dwilight is as much about man v. nature as anything else, and a cluster of confederated city-states with some organizing principles (tMP) and a shared common ancestry (Lurian Empire) sounds much more fun than a single homogenous blob.
PeL has its own culture, LN has developed a distinct one fairly shortly, and considering the people leading the push for Shinnen, I expect they would as well. Some of us (mostly in PeL) would LOVE to bring Fissoa into this. It's the perfect size and place.
