Having provinces on their own is one thing, but ability to make good trades is another thing. Imagine that Carthage has only one province with wood and me (Rome) take it in peace. If all the other provinces have their trade limits full, Carthage is doomed
It could be solved by intelligent AI trading combined with very dynamic (probably only short term trades) trade system. Another thing is proper diversity of resources, so player won't be able to cripple everybody by taking ie. all 10 provinces with wood. I simply hope that in the long run AI won't be crippled by taking few key provinces.