I just wanted to add, Mercantilism is particularly useful on maps where you will not be meeting many other civs - archipelago for sure, and continents when you may be alone or only with one other civ to start (depending). You usually don't even meet the other civs until well into the game. By that time, you've had mercantilism running essentially without any penalty. In any case, it is most useful early in the game while foreign trade routes give you +2/+2 commerce instead of the domestic +1/+1, in other words, a small tradeoff.