Well I'm not exactly an expert on spears, but they do have quite a decent amount of power, even if unwieldly. This is probably one of the reasons Halberd formations evolved, as cavalry dominated swordsmen and held their own against normal phalanx style spearmen. So cavalry were prefered, ans as a result a shield was less important as it didn't provide as much protection from all the weight of a knight in all his armor charging on a horse, the spearman would at least be knocked over and due to his unwieldly one-handed spear not provide enough stopping power to take out a knight. A halberd with out spears could not only be used as an axe and slashing weapon against other foot soldiers but a spear against a knight with more stopping power due to two hands being involved. Pikes were similar, being longer but not as effective against infantry weapons. Pikes were better against cavalry, but halberds more often won the day against swordsmen (this is not to say that they won it often, only more than pikemen).