How effective was the Pilum?

[The General]

I mean, why whould they use them if they only caused one or two casualties a volley?

Ugh, my javelins tend to cause casualties in the dozens (I play on huge settings). As Lusotani, the battles are just brutal in Iberia, with hundreds of javelins in the air and soon dozens of men on both sides lay slain...

I'm no expert, but the bow and arrow's main drawback was the arrow's lack of mass behind it, so it didn't have enough punch.

Longbows? Composite bows are quite effective too, afaik.

If you want to kill heavy armored units with pilums, position your troops on the right flank of them and throw the pilums from there, because they won't get the shield bonuses. Right?

Aye, rear is a good place from which to throw, too. Skirmisher cavalry, eg. Equites Campanici, especially can do nice morale damage to an enemy, all the while killing foes. Follow this with a charge to the flank/rear and just watch the panic spread...