This is why art has a credibility problem.
And yet, it's also an interesting working example of the Intentional Fallacy, the literary concept that a text can mean something that the author didn't intend it to (i.e., relying solely on authorial intent is the fallacy). The Intentional Fallacy certainly allows readers/art viewers to interpret a piece of work without being restricted by the question, "what did the author/artist mean by that?"
Of course, this incident takes the concept even further by questioning the artist's very definition of "art" and what constitutes a complete piece of artwork. If the artist does not believe that it's art, but other people do, and if these other people hold positions of authority in the field (i.e., have cultural capital), is it art? Mind you, the artist in this case also possesses cultural capital by virtue of being a university professor of art. So we have two sides, both representing powerful state and cultural institutions, debating the artistic merit of a slate base and stick of wood.
I could (and will) take this post even further down tangential lane by noting that there is a fine tradition of making art from fragments. The Modernists loved fragmentation, the breakdown of traditional language forms to "make it new" (see: Joyce, Eliot, Emerson, the Dadaists, etc.). Reaching even further back, the few of Greek poet Sappho's poems we know of are fragments. In the case of the Modernists, the fragmentation is purposeful and effective. In the case of Sappho, does knowing that all we have are fragments compromise the integrity of her work? Would we read her poems differently if we didn't know that they were incomplete? Do we bring a bias against incomplete works to our readings? Have readers always insisted on completion, or does this need arise from cultural and social forces?
I think there is a basic human need for things to end in a satisfactory (not necessarily happy) manner. Why else is there all this talk about needing "closure" or starting a new chapter in our lives? And maybe when real life doesn't end up as clearly delineated as we would like, we turn to entertainment to give us that sense of closure instead. Only, I also think that some of the best writing out there explodes the concept of unity. Does art imitate life, or is life imitating art?