In mainstream economics 'Marginal Utility' refers to an increase or decrease in utility following the consumption of an additional unit of a good. I fail to see what that article has to do with it?
Well, I suppose the utter uselessness of Californium, and several other of the items on that list illustrates the Diamonds half of the Paradox of Value (http://en.wikipedia.org/wiki/Paradox_of_value) note, the apparent solution of the problem, by Marginalism, is, to my mind, unutterable bunkum. Diamonds are precious because they are rare, but they are only rare because they are desired. Since they have no otehr use, it is their value that makes them desirable.