Sparta after all did not produce anything except warriors and the destruction of Athens. Athens on the other hand produced the greatest Art, Mathematics, literature, music, philosophers that the world had ever seen.
From the Rambam's point of view however there would only seem to be one justification and that is that it is a contract. That is whatever system people decide to live under has the force of Torah Law as long as it does not violate other Torah Laws. The reason is because any contract has the force of Torah validity. This applies to government also as is brought in Bava Batra.
Government of course is not the same as private contracts-. But all the more so that gives it a need and validity that goes beyond private contracts. For without government, no private contract is possible as Dr Epstein makes clear in his debate with Dr Michael Huemer.