Here's an elegant argument from Brian Boyd's new book On the Origin of Stories --
"As Richard Dawkins notes: 'natural selection is a predictive theory. The Darwinian can make the confident prediction that, if dams were a useless waste of time, rival beavers who refrained from building them would survive better and pass on genetic tendencies not to build.' Likewise with art: if and only if art were useless, more ruthlessly utilitarian and competitive realists with a lesser inclination to art would have survived and reproduced in greater numbers, and over evolutionary time their descendants would have supplanted those with a disposition to art. Societies without any inclination to create their own dress, song, and story would have ousted those that did have these things. Individuals and groups without art -- without shared songs and dance (including face-paint, scarification, tattoos, uniforms, emblems, flags, or monumental architecture), without shared stories and sayings (including myths, heroic legends, proverbs) -- would fare better than those with all these things, which art makes possible. But that seems never to have been the case. No human society lacks art, and the more successful societies have more art than ever before."
Does this argument prove that art is useful... or does it prove that arguments founded on evolutionary psychology can be used to prove anything...?
Note that logic identical to Boyd's can be used to prove that religion is useful.
What is art useful for? Boyd's line is that art fine-tunes our capacities for living -- it trains us, briefs us, prepares us...