Where do you get the idea that shields weren't an integral part of the panoply of a spear-armed warrior?

In pretty much every era from the Classical (Hypaspistai, Hoplites) to the Early Medieval (vikings, Anglo-Saxons, Franks, etc.) we have evidence of spear-and-shield-armed soldiers.

There are innumerable examples of spear-and-shield-armed warriors in art and sculpture from all over the Mediterranean, for example.

The Capitulary of Aachen, issued in 802-803 (page 9 of this PDF) for example, dictates that Frankish counts equip their foot soldiers with a lance, shield, bow, a spare string, and a dozen arrows, each. Notker's biography of Charlemagne - though full of embellishments - also notes that Charlemagne's foot soldiers carried shields, spears, and swords, all of iron.

In the Konungs Skuggsjá, a 13th century Norwegian manual for nobility, there is a section on preparing for war and a shield is mentioned as being essential.

Suggested reading:

DeVries, Kelly, and Robert Douglas Smith. Medieval Weapons: An Illustrated History of Their Impact

Halsall, Guy. Warfare and Society in the Barbarian West 450-900

EDIT: A link

Spears without shields (usually, but not always, two-handed spears) appear on the battlefield in a few contexts:

1. When the main long-range weapons are gunpowder artillery and guns. In this case, the benefit of shields is reduced, because they usually do not provide good protection against guns. Archery might still be in use, but if the main threat is guns, then protection against arrows matters less. Body armour might be retained for protection, depending on the effectiveness against guns, and the likelihood of close combat. Some examples of spears without shields in such conditions:

• Early modern pikemen in Europe

• Early modern spearmen in China (Qing Dynasty)

• Modern lance-armed cavalry (as late as WW1)

There are also cases where, despite the absence of guns, long-range weapons are absent or only a minor threat. One example is traditional Maori warfare.

2. When armour provides good protection against arrows (or the main long-range threat, if a weapon other than arrows). For spearmen/pikemen in close formation, it is often sufficient for the front few ranks of soldiers to be armoured. Examples include:

• Renaissance pikemen

• Kiribati warriors (protected by coconut fibre armour against sling stones and thrown stones and clubs)

3. When tactics provide protection against arrows. If your own archers can keep enemy archers out of effective range of your spearmen, your spearmen have less need for shields. If the spearmen can use mobility to either stay out of range of archers, or to threaten them, they have less need for shields. Fortifications, including field fortifications, can provide protection against arrows. Examples include:

• New Guinea highlands warriors. Where spears are used without shields, most warriors use a bow, and the battle is largely an archery duel. Spearmen stay out of range. If one side collapses during the archery duel, the spearmen will help pursue the fleeing enemy. (Shields were used by some groups, but not by all.)

• Cavalry.

Often, both 2 and 3 are used in combination. Both are important for soldiers other than spearmen who need both hands to use their weapons, notably archers and crossbowmen.

Why was the spear, often without a shield, so constantly used everywhere despite the threat of arrows?

Considering the above, when arrows were a threat, the spear was used with a shield. When arrows were not a major threat, and especially when guns reduced the benefit of shields, the two-handed spear without a shield became much more common.

There is, however, another context where the spear is seen without a shield: civilian use. This can be a spear as a self-defence weapon for travellers, as a weapon carried by bodyguards, as a duelling weapon. Essentially, this is a context where the threat from arrows is much less than on the battlefield. Much of the material of spear in European fightbooks, Chinese fightbooks and Chinese martial arts instruction, and Japanese martial arts instruction is for such civilian use.