What about when a couple with a dependent spouse (usually the wife) moves from one state to another, from a "community property" state to one that isn't? The marriage already occurred, and given the laws of the state where the couple married, they didn't feel a pre-nup was necessary. But now, say with the husband having finished medical which the wife worked to put him through, the national residency matching system is forcing them to move to a non-community property state. And the husband secretly isn't so sure any more that the marriage is forever, and throws cold water on the idea of doing a financial agreement (or more likely doesn't even have to, since probably neither of them is even aware of the different laws in the state they're heading to) -- so the wife is totally screwed, when hubby finds a cute young nurse a couple of years down the road, long after residency in the new state has been established.
This sort of thing really does happen to people, and it's high time that government be booted out of the marriage business. The traditional legal concept of marriage dates to an era when women were legally chattel, and were not able to enter into any kind of contract. That's how the government got to be a party to the marriage contract -- to protect the poor helpless females who weren't full fledged citizens. It just doesn't have any applicability to modern society.
Community property laws may still apply. Get a legal opinion from a competent lawyer in the state where the parties reside.