I've found that they're great for keeping your opponent's general out of the fight, particularly if they're a leadership general. Either they'll stay out of range, where they can't help buff their troops; or they'll run around all over the place and get distracted from the battle. Either way, they aren't using stand and fight, which is a huge blow to their troops (especially if their army is a loan sword spam or something, with low morale). The trick, I've found, is only to fire a single volley at the opponent's general at first, as most people don't realise how little ammunition they have. If your opponent assumes they have 10 ammunition, they'll be less likely to just risk it or try dodging. However, despite this I personally don't use them - I just think 2 katana cav or so are much better value.