Perhaps I'm being too much of a Roman fanboy, but the pilum seems to be quite the dreadful missile weapon, one I wouldn't want to see hurled at me before I closed with the enemy.

Why do we not see more use of this weapon in the dark/middle ages?

Edit: To perhaps interject my own speculation (I'm a fan of history, but no historian), I would guess that perhaps the pilum was dropped because of its weight, extra cost, training and perhaps losing of Roman Legionnaire tradition.