Avoid that law like plague. Nothing good comes out of it, the demesine bonus it provides is not enough to make up for the territorial losses and probable succession crisis that will follow, almost without exception, IMO at least. What usually happens in my game (where i dont get the opportunity to change it) is id pick a group of provinces that i will hold. As time goes on, i invest money in them, and since im usually the overlord (main title holder in my realm), i usually get the most money, which means i hold the most developed provinces (most troops belong to me directly). Then my guy dies, and all of a sudden, the 12 provinces that i owned are gone, and im left with like 2-3 (depending on number of children that inherit i think), all of them having claim on my main title, and all of them plotting to see me dead. Its a bad, bad situation.