From my point of view, the "easy to produce and easy to train" arguments are only part of the success of the spear. As Pratchett put it: "Everything dangerous happens at the other end". A club would even be easyier to produce and train, but a peasent would have to come close to the enemy to do something against him.
About the bamboo spear: I think what was meant was this martial arts spear, that bends a lot and is wielded more like a cross between chain and blade, sometimes to pierce, sometimes to slash (as in "Hero", for example). At least the rules make sense for this kind of spear.
Wasn't the pilum the "spear" whose long metallic point bend when hitting the enemy to prevent it from being thrown back? Something like this would be quite useless in close combat.
And we have to make sure, about which period of time we are talking here. The republic army had more use of spears as a reach weapon, while Marius Asses where more into the thrown pilum thing, as far as I remember.