If players wanted their own world, You can press a button, not many people know of this, It's found on the title screen, however everyone seems to be blind to it. Its called "SinglePlayer"
Last i checked, Its why Minecraft got popular in the first place.
I don't really see the point.
The whole idea of MultiPlayer, Is of multiple people to easily interact in the game world, What better way to do this then Survival or Factions?