It's not terribly economically efficient for most resources; most that the computer goes after just aren't worth that terribly much. So what if he's trading wine in France and making what? Dozens of florins a turn? Some of the stuff they go after, they'll be lucky if they ever recoup the investment in their merchant in the first place.

And I almost never see the AI go after any of the really juicy merchant areas, Timbuktu, Arguin, the Americas... Hopefully in a patch or two, the AI will get more active on that. Not everyone should head there, but it'd be cool if playing a long style game having your next couple biggest rivals also pursue the new world. Right now it mostly seems like a fun diversion for single player games, instead of opening up a new battleground and new form of warfare.