The installation at the Tate Modern (below), Doris Salcedo’s Shibboleth provides an excellent example of rhetoric standing in for, or justifying, the art. This is excerpted from the publicity material, which I suggest reading in toto just to get the full effect:

Salcedo is addressing a long legacy of racism and colonialism that underlies the modern world…” The history of racism,” Salcedo writes, “runs parallel to the history of modernity, and is its untold dark side”…. Our own time, Salcedo is keen to remind us, remains defined by the existence of a huge socially excluded underclass, in Western as well as post-colonial societies…”

Hullo, it’s a crack. A crack. A break in concrete. The artist’s intention does not change the experience, which happens to be one that leads to strange parental behavior. But if you insist on metaphor, it could represent any disparity—including the one between those who are willing to shell out $50 for a Duchamp T-shirt and those who aren’t.