I only use them in early game, when you don't have alot of koku and your opponents are not very teched up. They can defeat the light cavalry units and so should be used against them. They are very vulnerable to missiles so keep them out of range. If your opponent does not have any cavalry, use there high speed to flank the enemy units once your Yari sam has engaged them. Can also be used to chase down routing troops. But you are right, other than that they are best used as garrisons and also for the bridge bait tactic.