The idea behind this is IMHO that devs want you to play -your nation- , with an according strategy, and not a mix. Armies as a mix of many factions' units almost always worked badly in games, in my experience... Players just start to create the maximum - cost efficient army and when one is determined, everyone tries to get to that one and the game becomes standardized and boring.
I really like this as it works in Dom3 - you are the god of /this/ nation and /those/ are your followers - except of course if you brainwash and (master) enslave your enemies
