In an essay about Bach Cello Suites in Richard Tarushkin's The Danger of Music and Other Anti-Utopian Essays, he refers to the phrase "Tonschönheit ist Nebensache" (beauty of sound is secondary) as Paul Hindemith's motto. He does mention (In an essay about Hindemith) that the phrase comes from Hindemith's tempo marking in the fourth movement of his Opus 25, no. 1 Sonata, "Rasendes Zeitmass. Wild. Tonschönheit ist Nebensache," but I believe that Mr. Taruskin jumps to a general conclusion about Hindemith's feelings about expression that may not be appropriate.
As you can hear from the performance below, the movement can be played with wild abandon, scratching the instrument like crazy:
But I believe Hindemith's suggestion is specific to the one minute and 51 seconds it takes to play the movement, and not to his music in general. It is a minor point, I know, but the word Nebensache also means a minor point. I believe that what Hindemith is telling the violist is to play with abandon and not to worry too much about making a beautiful sound.
I have heard it played beautifully, by the way.