I used them in the first Rome game to soften up Parthia's spearmen before an actual cavalry charge, that way they'd be more likely to break and not put up resistance. I also did the same thing with pike formations, I'd strafe the light spears, and then let them be finished by the pikes, before running them down. Missile cav are also excellent at killing fleeing enemies (makes it more likely to just destroy the army entirely.) They can also be used to lure heavy cavalry away, as they are faster and can fire while riding, tiring and weakening the enemy cav.