Everyone has an opinion on Common Markets.
With that said, I have no use for them.
The more products you can place on the world market, the better your profit margins will be.
You can contract good and services with yourself if needed.
If your operation is not big enough to contract with itself i.e. lack of certain industries, then maybe a CM is good for that.........
They also increase your country score and can be helpful in increasing your country's rank, however, you can reach #1 without them.