Using a spear against a shield can be an effective means of countering the defensive power of a shield. Many soldiers throughout history carried a javelin, long spear or short spear in addition to their sword and shield. They would use the spear just as they closed with the enemy before drawing their sword for the remainder of the battle. If a long, heavy spear is stuck in your shield, it can make the shield difficult to wield, sometimes forcing the defender to drop their shield entirely. The Romans in fact developed this sort of tactic to a high degree. They began using a short throwing spear called a pilum which was thrown at the front ranks of an oncoming enemy en masse to cause disarray in the front lines just before closing to sword-distance. They eventually developed a special form of pilum with a very soft, short, iron shaft just behind the head of the spear, so that when it stuck in an enemy's shield ( or his armor), the weight of the wooden shaft of the spear jutting out would bend this short, weak length of soft iron, disfiguring the spear and making it unable to be removed and thrown back at the roman army. it was a disposable spear designed to be unusable after impact. brilliant!
additionally, spears, and then eventually lances were utilized by cavalry in charges specifically to break walls of shields. when an armored horse and rider are charging at full gallop, all of that tremendous weight and power are transferred to the fine point of their spear or lance, transferring on impact into a shattering blow, capable of breaking even the sturdiest line of soldiers. Thus in some instances, the spear is the MOST effective means of disabling the defensive power of even the mightiest wall of shields.