Yeah, lot of good reasons for using missile troops mentioned in this thread. Although I would say they are a lot more useful on offence than defence in pitched battles. As you can send them ahead to do damage before your main force arrives, and expend a much higher percentage of their missiles before real contact is made

Also a couple no-one else seems to have posted. Use them as a screen in your own army to get those undisciplined barbarians constantly charging at one of your least useful units miles away from the rest of the battle.

Also, they can be used as fodder in your own advance. The enemy ends up doing all their damage to these units which are cheap to replace - usually trained in low MICs etc - while your main heavy infantry/cavalry groups can often get into combat - or at least in a position to throw their javelins - having acrued very little damage.