Quote Originally Posted by Gelatinous Cube View Post
If you have The Old Gods, the new tech system relies on Duchies to generate tech points. Basically, if a county isn't organized into a Duchy, it won't get any more advanced without sending the spymaster to steal tech from someone else.

So, if you can at all afford to do so, it is a good idea to create vassal duchies. Kings are entirely optional, and I tend to go without them, but some people prefer them for organizational purposes. In the end, you're going to have a civil war one way or the other--the plus is that you get to choose what it will look like well in advance.

Without the Old Gods, the biggest reason to create duchies is vassal management. Yes, Counts are easy to control, but what can they do? In a civil war they'll get annexed by your neighbors. With High Crown Authority or lower, where vassals can still wage war, counts aren't going to get you very much new land. The only plus is easy squashing during times of rebelliousness (like during the early reign of a bad king).
Good call on both the tech and annexed rebels, I forgot these points myself during my explanation.

Civil wars are almost impossible to avoid, but with high diplomacy you can usually prevent them from being too terrible. Being able to choose your heir with Elective (as in the HRE) is also a good way to ensure a smooth succession.