If you are familiar with the current pending Supreme Court case DeBoer v. Snyder (which for the Supreme Court was combined with 3 other cases) you'd know in many cases you can't just appoint any guardian for your children upon your death. Basically the facts are: a lesbian couple has 4 children - 2 adopted by one woman and 2 adopted by the other. All children have been raised together as brothers and sisters by both women. No children we ever being raised by only one parent. They found out that they couldn't guarantee that the other parent would keep all 4 children in the event of the death of one parent. Under Michigan law, in their case, the courts would be free to appoint guardianship of the deceased's two children to whomever they choose no matter the paperwork the couple drafted beforehand.
http://www.nytimes.com/2015/01/25/us/one-couples-unanticipat...
>Their close call with the truck that day in 2011 led them to a lawyer, Dana Nessel, who advised them that she could draw up guardianship papers, but that they would be nearly worthless legally. She urged them instead to file a federal lawsuit challenging the adoption law in Michigan.