sigh i stated all the uses of fire arrows along time ago. now its lost. oh well. uses:
good for scaring elephants and scythed chariots, can tip a unit into rout what ludens said, and keeping a unit into rout by shooting them when they are routing.
penalties: takes longer to reload, and is less acurate so it kills less. (though i wish fire arrows had a bonus against armored units or kills more)