Unfortunately, they both ignore an important property law reason for getting an official state-sanctioned marriage: it gives you the right to own property together in tenancy by the entirety. This form of concurrent ownership has two advantages not available otherwise. One co-tenant is not allowed to sell their interest without the other's consent. And should one spouse become insolvent, creditors are not allowed to seize the joint property to pay off her debts (though they still can do so if the debt in question is a joint debt owed by both partners to the marriage). So if your significant other tends to run up lots of debts that he can't pay, or you worry that he will sell his share in your jointly owned property to obnoxious third parties, you may want to get married. Assuming, of course, that you want to stay with such an untrustworthy spendthrift at all. Romance, thy name is property law!
On a slightly more serious note, I don't see any good reason why the right to own property in tenancy by the entirety should be limited to married people (and, in some states, participants in same-sex civil unions). Yes, that is the traditional common law rule. But I frankly don't see any good rationale for it.
UPDATE: I should note that tenancy by the entirety is permitted in only about half the states. So if this is you main reason for getting a state-sanctioned marriage, you should check whether your state allows it. Some 19 states and the District of Columbia allow tenancy by the entirety for the ownership of any type of property, and seven allow it only for real estate. See here for a list. Fortunately, my beloved Commonwealth of Virginia does allow it, and my fiancee (yes she's a lawyer too) has already suggested that we might want to convert our new house to this form of concurrent ownership once we tie the knot.