Honor, speed and defense.
1) anything that dies creates honor...honor makes it possible to defeat large armies or high level forts without fear of going below 50 morale
2) they go 1.5 speed man! You can pillage and be home 2X before the slow army with siege makes it home... think about where you can use the speed
3) To be thorough, although I cannot find it in the new manual, this was true in V4 as you know, in defense, all yuo want to do is stay alive until the enemy dies or runs. The javalin men spread more horse to wings for flank attack. In flank attack, horse attack with 1.5X attack. It is harder to kill an army with all horse on wings, than army with many horse in the center...especially if you have a mix of light and medium and elite horse. Lights fill wings before elite horse.
Honor is the difference between small time bully and creating a "death star" account which can reach out past even 3X for a kill.
He he he. I know all those things but they work in V4a. In my current v4a era i have almost 3000 moral so trust me i understand fight logic.
But mind you i wrote "at this early stage". Very soon we would get into heavy units and than into elite. Not much fight is done till people research elite.
By the way i never EVER pillage with spears. Small pack of archers would blow them away and almost always people keep some extra archers in forts. If you ask me pillage with spears is suicide (unless some extreme noob attack you)
Since V5 have extra unit for nomads that eliminate any advantage javelin could have i wonder what's the meaning of building them. Also i couldn't read anywhere if javalins would spread cavalry to wings.
I don't know if there is attack bonus for cavalry going on wings etc etc.