There's actually a very nice theory around to explain why human brains have surplus capacity. As you say, if brains evolved for survival, then it's puzzling that we are better at singing songs than at lighting fires without matches, given that the Pleistocene was a 2 million-year camping trip with not much karaoke.
Geoffrey Miller's answer, the fullest version of which is laid out in his new book, The Mating Mind, but which he's been on to for almost a decade and which I wrote about in my book The Red Queen, goes like this. We've been using the wrong Darwin book. Charles Darwin's Origin of Species (1859) is all about natural selection--survival of the fittest. His Descent of Man (1872) is all about sexual selection--reproduction of the sexiest. He thought the latter was just as important an evolutionary force as the former, but later biologists just ignored it. Sexual selection is all the rage again now and is undoubtedly the right explanation for all sorts of extravagant, wasteful, and capricious things in the world--mockingbirds' songs, peacocks' tails, daffodil's flowers, and such like. They are used for competing to get mates. Sexual selection seems almost to specialize in producing "useless" ornaments, so maybe it produced the human brain. As Miller argues, natural selection without sexual selection is like industrial production without marketing.
So Miller runs through lots of evidence that the main function of art, wit, poetry, creativity, music, political ambition, and even selfless morality seems to be to show off to potential mates. The link does not have to be conscious, of course, but it's amazing how often these features seem to show the right characteristics to be sexually selected: appearing at the age of mate choice, resulting in sexual success, etc, etc. To put it bluntly, we have big brains because back in the Pleistocene people with big brains got the best mates.
It makes you feel a bit weird about your work, doesn't it?
I agree with you: It's wonderful that grown men and women are paid to ponder things like this.
It's been great. Thanks very much for the chat and thanks, too, to the people who've commented in "The Fray" and directly. I learned a lot.