"The Shortspear is a lighter version of the Javelin that deals less damage but can be thrown faster and farther."
The Shortspear's damage is velocity based and as a result, sprinting and then hurling the spear causes it to fly farther and deal signifigantly more damage. With the Shortspear, this is often needed in order to preform one hit kills from a max health foe.
Compared to the other Javelins, the Short Spear leans on being a versatile melee weapon more than a true ranged weapon, as the weapon's high speed gives the archer very quick stabs and a very fast throw that's practical in close quarters. Thus, this "javelin" is strongly recommended over the others in arena-like situations and the Free-For-All game mode where you'll need the extra survivability in close combat. Alternatively, if you simply enjoy being an archer on the front lines or like to play archer aggresively, look no further than the Short Spear.
Be sure to mix up the combat with kicks, faints, and/or the (weak) overhead swing "shield-bash", as predictability can hinder your offense if your foe is skilled. Alternatively, use your quick throw at mid-close range to catch them off guard, especially if they lack a shield to block the blow.
The Short Spear's weak throw is offset by the weapon's increased accuracy, making it an ideal way to finish weakened foes whom you've/your allies faced in close combat, particularly if they're running away.
It should be noted that the Javelin type weapons have had their recovery time increased since the Chivalry: Medieval Warfare Beta. It will take a few seconds to bring up your next Javelin or secondary weapon after a throw. You will be helpless against a close-range assailant in the meantime, reducing the effectiveness of the Javelin class. The Shortspear also has the same damage threshold as the Saber and Cudgel when used so consider switching to your secondary weapon when you feel that it's unsafe to continue fighting with your javelin.
It is possible to negate the long recovery time by quickly switch to your secondary weapon, and switching back to the Javelin.
Note that this Javelin doesn't have the same type on the Archer's back, and also as you can see on the ArrowCam you can see that the Javelin is there instead of the Shortspear
These may be patched in the next update.