Romani don't have really all that great missile units. You can often get balaeric (SP?) slingers as mercenaries in many of the theatres you'll be fighting in, which are pretty much unstoppable, with 2 units of them and legion or 2, you can win heroic victories with only a couple casualties it seems. But as for faction units, accensi are the worst slingers in the game, though with experience they can be decent. Leves and later, velites, I like to have just for realism, and they can really wreak havoc on unarmored foes, I've often had javelin armed units reduce enemy levy units by 1/2 on level terrain by expending their volleys, though I play on huge unit scale and missiles are more effective. But when it comes down to it, you can win battles easily with just heavy infantry, though I still like to have missiles, and they can be effective in reducing the enemy so you don't have to work your infantry as hard.