Mercs can be enough to change the whole game from the start.

One way to use them, works well in VI, is to buy a bunch early in the game before any neighbors have big armies, and to keep them from getting them. Since they are cheap you can buy enough to make a big difference in stomping the surrondings. Make sure that in any attrition they are up first to save your real armies for down the stretch.

If you conquer enough quickly you can keep them, but if things go poorly you may have to ditch some. At the very least they will have hopefully been successful in taking your enemies down several notches.

Not to say this is really an ethical approach..