Longbows? Composite bows are quite effective too, afaik.
Longbows didn't exist back then, at least not outside of India (I think). Anywho, the effectiveness of an arrow is dependent on mass and force. The Longbow has a massive, actually heavy arrow and alot of force and velocity, which explains why it was so effective against armor. The composite bow's arrow is not nearly as big, but it tends to make up for that in it's snap force.

I wasn't much thinking about the eastern scheme of things, but the west had fairly shitty archers and bows in general. The composite bow was a good deal better than predecessors, but at the same time, if it was so good, why didn't every soldier carry one? If that was true, they could level enemies in minutes. However, armor and shields limited their effectiveness severely. Javelins, especially things like the soliferum and pilum were useful tools for tackling armored hulks, given the weight of the missiles.

And remember, it took ALL DAY for the Phalavans to defeat the Romans at Carrahae with volleys of arrows, where as in the Pelopenisan War, Spartan hoplites took on a number of Athenian peltasts and Javelineers on a certain island (can't remember ther name) off the southern coast of greece, and were defeated in several hours, then finished off later. The spartans were heavily armed and armored, but javelins clearly helped get around that.

As for the psychological edge, its difficult to represent that in RTW, but yes, it's really frustrating and irritating, and probably frightening, to not be able to fight back. But still, javelins still have the potential to cause a hell of a lot for damage than one or two casualties.