I disagree with this thread on one point: for me, dynastic inheritence is almost as common as political annexation, depending on which nation you are playing.
A classical inheritence is Georgia being absorbed by Byzans about ten to fifteen years into the game, which is one of the reasons why I choose not to play as Byzans. Sure, it opens another front against the Osmans, but I've never witnessed a people so prone to revolutions as the Georgians. It never results in them forming their own nation, though. They seem to be perfectly happy with rising against me and then keep their two provinces for fifty years or more without declaring independance.
The most dynastic inheritences I've had in one game is two, separated by only about five years. I was playing as England in the IGC and inherited Hannover around 1500 (England inheriting Hannover is something that happens every second time I play as England). About five years later - and this is more or less unbelievable - the king of Austria died and gave me his country, including Bohemia, Lothringen and most of the lowlands.
Even if that sounds nice, it was decidedly not. The Austrians were almost Georgian in their uprisings (or maybe it just seemed like it because there were so many provinces?) and because the alliance Austria-England-Spain had just been at war with France, the Austrian army was depleted. And since France had... Luxemburg or something like that, I couldn't do much about the rebellions in mainland Austria.
I estimate that dynastic inheritance happen to me every tenth time I play, but that number varies a lot depending on what nation I play. The Osmans, France, Spain, Portugal, Russia and Venice have never got any inheritence, whereas Byzans, England, the Papal State (have got Tuscany and Milan), Sweden (The Teutonic Knights, strangely enough) and the Netherlands (Mainly Hessony, although also Cologne) gets them regularly.