Performing a piece of music in a high-pressure situation is exactly a professional musician's job, so an audition is quite appropriate. Another analogy could be to ask a professional composer to play one of his pieces on piano rather than listening to a competent orchestra perform it--you're using one thing as a proxy for another. Many composers would ace it--they're great players. Some aren't, though, so you'll hire only composers who happen to be great performers. It's a dumb analogy in another way, though: you only need to know whether you can work with a composer, their work should stand on its own. I suppose if we engineering types all wrote code that went into public repositories the same might be true of us, and we wouldn't be asked to perform like trained monkeys.