Quote Originally Posted by rvg
If fully taken advantage of, it will bring in more money than you could ever hope to spend, making the game impossible to lose (it's difficult to lose battles if you are not VASTLY outnumbered, as the computer is a moron in tactical combat).

I highly recommend downloading the XL mod. It greatly reduces trade income, although it's still worth setting up a trade network. It also improves the value of land and farming upgrades, which helps the AI-controlled factions keep up with you.