One of the main reasons I feel that civil wars/re-emergences should be reimplemented would be to help prevent "blitzkreig" style expansion by the player. If they want to expand they would have to make sure they had standing armies throughout their lands at certain intervals, otherwise, a re-emergence or a civil war could entirely gut your empire from within. The main reason I normally get bored with a campaign is because I'm able to simply steamroll any and all opposition and only leave behind a few rebel hunting parties here and there. If there were entire populations of an opressed people rising up against me it would likely take a bit more than my standard hunting party to deal with them. The same goes for a civil war. This way I might actually get to reach the Marius Reforms before the game is over.