The most extreme form of this logical argument is "suppose what would happen to governments if teleportation was cheap and available tomorrow?" The consequences are obvious. All labor would immediately move to where they could be paid best and taxed least. Governments would crumble left and right, as the tyranny they could inflict would vastly reduce immediately. Ron's argument about the tremendous social upheval this would cause (and likely war) is certainly a reason to be wary; but when things are a matter of policy rather than technology people tend to fight harder to oppose it. That's really the best reason to oppose totally open borders -- under a state system the revolutionary violence that would result is probably not worth the potential benefit.