The Spear Throwers of Rohan are exceptional marksmen that can hurl spears. They have the particularity of having the strengths of pikemen from range. They are the only spear-throwing units in the game besides the Dwarven Hunters of Ered Mithrin from the Iron Hills.
The Rohan Spear Throwers have access to the following upgrades:
Banner Carrier - This upgrade can only be given to a level 1 battalion and will promote it to level 2. At level 2 the battalion can regenerate units when out of combat. Cost: 200
Heavy Armor - Increases the armor, hit points and damage resistance of the unit to ranged damage. Changes the unit's armor set to EdainBowPikeHeavyArmor. Cost: 300
Forged Blades - Increases the damage of this battalion and provides a damage bonus against other infantry (swordsmen, pikemen, archers, etc...). Cost: 300
Spear Throwers come in battalions of ten and are somewhat expensive, but worth their cost. They are quite a unique unit because they have the advantages of pikemen but at range, so they can be used to kill cavalry and monsters from a distance. They are great for covering units that are very vulnerable to trample damage, stopping an enemy cavalry charge before it can reach you. For this reason, they should not be used to replace Yeomen Archers since they aren't good at dealing damage against infantry but should be used alongside them. Their Mighty Spears ability can allow them to be effective against other units too, even if just to slow them down by knocking them to the ground.
At the moment, the Spearthrowers are bugged as they will not automatically attack nearby enemies like other archers would. However, one or two battalions (if micro managed well) can be targeted to hit specific parts of the enemy army, allowing you to disable their strongest troops while your peasants and cavalry can clean up the rest.