The Nodachi Samurai and Sohei (Monks) were added to the game for gameplay balance reasons.

Historically the Nodachi Samurai (not the same thing as the Odachi) were used for the same purpose as Naginta. From a gameplay perspective, history does not always work, which is why we have battlefield sword units in both STW and MTW. This ensures that the "swords beats spears beats cavalry beats swords" (RPS) system works as intended.

Despite this, Nodachi Samurai are certainly closer to reality, as "anti spear flankers", than the various sword units in MTW.