A lot of people will go all frog-in-a-blender about the idea, I reckon, because it seems to lack romance. But I think it's funny that people enter marriage as if it were not a contract, but just two people walking hand in hand through fields of flowers. Tra-la-la-la-la!
I think most people think of their marriage vows as poetry -- they really mean the promises they're making, but mostly they're approaching those promises as a piece of art, to express all the schmoopiness they feel inside. That's great, and it makes for lovely weddings, but I think it's a really good idea to get those OTHER vows in writing -- the serious, left-brain vows, about specifically what each partner is willing to do, be, and have in the relationship, and also what (s)he is NOT willing to do, be, or have. And outlining the specific agreements in the relationship that most people leave unstated and that cause all kinds of trouble when people break them without even realizing they had made an agreement! And the consequences of breaking those agreements.
Of course things change over the course of the relationship and the contract will probably need revising and renegotiating if you stay married for any length of time, so it's not like a contract with the devil.
