Of course they do. But we live in an era where we have to prove *everything*.
Just like it's obviously beneficial to read aloud to children, and it helps to let them choose the books.
Really, have we lost all sense? When we have to go around constructing experiments to prove the worth of the arts and other public goods, I get even more apprehensive about our society.
Meanwhile, our president wants to hook everyone up with ebooks, just like Comcast wants to provide low-cost internet to households in poverty. There are such better ways to spend our money, but I guess it's up to us to prove it.