Decentralised vassals for large entities is important, especially the burning legion which is practically everywhere in the world, right now burning legion's forces are in one area of the world and yet it's provinces can be attacked anywhere, by having vassal off-shoots, they can spawn their own armies, protect themselves and eventually the main army will reach any attacked province.

Stormwind i felt was too powerful having direct control over everything but in reality in warcraft, it's only as powerful as it's lords