Well in the balancing of it; maybe using pillaging to gain an army would be more factoryish, but if every time you conquer a province you gained an army it can be exploited (SP especially; but MP is my thoughts) by allowing provinces to be defeated in order to retake them and X number of new units.

Perhaps it could be used akin to a Pan; they flock to a Khan in a territory that was recently overtaken (Up to ~5 turns previous).

You don't want to make the new units totally worthless so there must be a limitation on how they are found.