I think that complicates the issue and opens up a new can of worms. Say a planet is like Earth that has two hundred or so nations--one or some want to join the Federation, but the other nations do not or rather join someone else. I don't think the Federation would force themselves onto a planet that divided (as already mentioned, the situation on Kesprytt III was seen as an unusual case by Picard and was only an evaluation to see if it was possible).
To avoid such problems, the existence of a planetary government really has to be used as a yardstick for determining just how unified a world is. Otherwise, then current day Earth would be ready for Federation membership if only some nations fit the criteria while others do not and really are far from doing so.