I like games to be challenging, I don't want them to just hand me a lottery prize for pressing the Royal Marriage button. As Portugal I got a PU with England, as Hesse I got a PU with France. That's half of my games I'm able to acquire a major power with no effort at all. When a major power gets taken out of the game like this, the world becomes emptier. It doesn't colonize or do anything else - other than supply you with armies. There should be an upper limit on the number of provinces that can be inherited - maybe those nations automatically break the PU when the first ruler dies.