This is a follow-up to the question asked in the Rules Forum.
The main use of a butt-spike on a spear was that you could stab a man on the ground in front of you without having to entirely change your grip. You would not swing the spear around to use the butt-spike against enemies in front of you. Instead, you would be able to stab a fallen enemy at your feet with only a minimal amount of grip change. This is especially useful in a formation like a phalanx where it would be impossible to swing the spear around to use the head, and saves the soldier from having to draw his sword or dagger.
Also, Kyrian is (mostly) right about javelins as a whole. While javelins were a more ubiquitous weapon than he implies, they are generally a skirmishing weapon, and in that function were used by nearly every nation and culture. However, they were generally not expected to penetrate heavy armor. There are of course exceptions--the Roman pilum, the heavy throwing spear of Homeric Greece, the throwing spears of Roman cavalry. These are not, however, truly javelins, which are very light, relatively long range weapons, but heavier throwing spears, a distinction that is not made in our rules. By the period that the majority of our people represent, however, he is quite correct about javelins/throwing spears being a rare weapon of war, a fact that is in part due to improved bow technology and availability.
In truth, Belegarth is really a sport of skirmish battles, and in this setting, I think that javlins are very appropriate and historical, even if the physics and safety of foam fighting dictate that they must operate differently.