I think a worry people have with splitting African and Asian countries is that once it happens a few times, it will motivate further independence movements and cause wars, genocides, and instability.
Yeah, that's I think one of the primary worries... not so much that any particular division is bad, but going back to a world where borders can and do change with some regularity would lead to far too much instability. The idea that borders are divinely appointed, sacrosanct entities does cause a lot of grief, especially when those borders were drawn by idiotic Europeans, but it does keep countries from going to war every time they see something on the next side of the border that they'd like to have.
