Slitherine

These units seem worthless. You can be running around the enemy rear but their attacks don't do anything. They almost seem like liabilities in that they can set off chain reactions when they inevitably retreat. How are these units supposed to be used?

If they're not doing anything I'd suggest you look at their ammo .. bottom panels .. if it hits zero then they're scavenging off the floor and a lot less effective at ranged/impact fire.

I'd also try to pull them out before they get drawn into close combat. They're skirmishes .. hit and run .. and don't really excel in melee.

They are far from worthless. Light javelins are good for screening your main troops and neutralising enemy skirmishers. They will usually beat slingers and foot archer skirmishers in melee. And they are good for holding rough terrain and holding up enemy units who want to cross it quickly. Light horse javelins are essential and I always have at least two of them if available. They can hunt enemy foot skirmishers effectively and they can be lethal against disrupted and fragmented main enemy units, particularly In the second half of a battle when the formations start to break up. They are also good for attacking routed enemy leadership units (to make sure they don't rally and return to the battle).