I think the closer an enemy culture is to your own the greater the chance their MIC will work for you. Romans share their MIC with the Epirotes, Eastern Greeks like Baktria, AS and Ptolies have one. The neighboring states of Hayasdan and Pontos share theirs and so on.
The Sabyn share theirs with the Carthies who resemble them mostly in general culture. The Celtic factions of Gaul and Briton do the same. That's why they can recruit from eachothers MIC's.
It also presents a problem when your neighbor comes to call. If his culture is similar he can churn out good troops from a settlement he conquered from you, where you spent valuable mnai to upgrade your MIC.