Jav cav can be great early in the campaign. You have to be quite tactical in using them though. Don't let them throw their javelins until they can hit the rear of the enemy unit. Bait the enemy with a couple of units of jav cav and split them up into single units around the battlefield, then throw javelins at point blank from behind to weaken the enemy. Tire them out until they're exhausted and surround them to reduce their morale. Charge from multiple sides at once and rout. Keep doing this on repeat and you'll take down large armies. Jav cav is much better than most light cav against the AI. Take this battle against the Greeks fielding Militia Hoplites, Rhodian Sligers, peltasts and some Greek Cav for instance:
The general does the hard hitting of course, but the militia cav are more than adequate to rout early enemies.
Generally, attack lightly armoured enemies with them, always flank, stay well away from elephants and chariots, and group charge at weakened isolated units.