Originally Posted by
Spariel
Public Order is the main reason, but there are others. A big one is autonomy - ie the % of maintenance and income borne by the state. Allied states have a higher autonomy, maybe 50%, whereas state chains provide much higher integration, reaching full integration by tier IV or V. This means if you want to make a region heavy on maintenance, eg temples, academies, recruiting grounds, etc., you should aim for lower integration. There has also been discussion about making some units unique to allied settlements, but that has not been implemented.
Edit: Forgot; tributaries give you more income than any other chain at the expense of less culture than state and less PO than Allied.