With a few archer units you can diminish the key units in an enemy army into a more manageable force, not only the AI but many of the others as well. The AI seldom fields elite armies, so killing off 66 % of his limited elite swordmen and his cavalry units really counts towards your victory. Even if that means 20-30 kills for all arrows spent, I prefer it to sending some of my units head-to-toe with them without all the advantage they can get.