Actualy, exclusive trade rights give -25% to trade value of province, and 66% to production. Exclusive trade rights mean you give a trade league COT, trade value of those provinces, meaning, you don't get trade from those provinces. But if you don't trade, and don't have COT, it actualy helps to have this modifier. If you have COT, and those provinces income would come to your own COT, you loose your trade center a certain level of trade, possibly decreasing taxes. And if you trade in your own COT, but not in the league that offer you those trade rights, you loose some amount of trade.
If you are trade league, and you cannot get a country into the league, giving it trade rights, increase your COT value, and so, it grant your COT, extra tax and income if you trade in your league COT. But if you would get income from those provinces anyway, it does actualy decrease your trade, so it does not worth it.
So simply thats it.