Your Roman pilum ranks alongside the English longbow as one of the great weapons of warfare.  At school, in Latin, we translated it as “spear”, which does it about as much justice as calling Beethoven a “pub turn”.
It was around 6’ long, the heavier shaft accounting for 4’, with a narrower spike of around 2’ at the business end.  The tip of the blade had a pyramidical point.  As a killing machine it was a masterpiece of design.

It could be thrown conventionally like any javelin but that was not its best usage.  It really came into its own as barbarian hordes charged the shielded ranks of a legion.  When thrown from around 20 metres the heavier weight of the shaft would propel the blade through most enemy shields.  So now with a 2’ spike poking through his protection the attacker would either be dead, wounded or, at best, severely encumbered.  He would then start to realise that his problems were only just beginning. 

Because of its pyramidical tip he would not be able to free it from his shield in a hurry, and hurrying is a feature of warfare; thus denying him the opportunity of chucking it back.  In any event, the pilum usually collapsed at the joint of shaft and spike (scholars are divided as to whether this was by design or a happy consequence).

Moreover, the weight of the shaft, whether still rigid or floppy, would be pulling his shield down disadvantaging him somewhat.  He would now find that not only was he encumbered by a 2’ spike on the safe side of his shield but also that the shaft was now snagging the ground in front of him.

He might decide to abandon his shield completely, opting instead to do a Braveheart type of war cry and throw himself onto the ranks of legionaries with a porcupine of spears poking between their scuta (shields that is, not scrotums), pretty soon finding that a bit of blue paint was not his best protection.  Alternatively, he might decide “Bugger this for a ludo militum. Abeo.”