I've found missile units to be a great addition to my Roman legions. I've used Balearic slingers, Numidian archers and Cretan archers to devastating effect against the Carthaginians (Numidians) in nothern Africa and in Iberia, and am beginning to outfit my forces in northern Italy with Iosatae in preparation for the Gaesatae horde. Do not underestimate their value!