Headphones have a function of reproducing sound. Are the different signatures of audio gear just different implementations of design, or are they particular aesthetics or interpretations of sound? If we regard the function of a headphone as purely to reproduce an audible sound, then we would all be using the original Beyerdynamic. If we regard the function of a headphone as the reproduction of sound in a 'nice' way, we are already slathering on layers of subjectivity and interpretation. Even in this case art and function are interlinked. We call this design.
Sometimes it was said that Steve Jobs was a great artisan. Is this because he made functional design? Can function itself be so elegant that it becomes artistic?
The function of art is to communicate, to evoke, to question, etc. There are effective ways and ineffective ways to do all these things, so art can indeed be assessed in terms of its successful function. We perform this assessment of function all the time. Every movie review does this. We do this every time we read a book and tell a friend, or press the "Like" button on a Youtube clip. The wonderful thing is that because this is entirely subjective, we can have endless arguments about the merit of an artwork in the same way we can have arguments about the merit of sporks or a Ferrari or the Abyss.
Just because the message communicated by art is sometimes difficult to recognise or discern, does not mean that art is an inscrutable thing with no function. I think the attitude that art is non-functional is the kind of scientism that makes this entire topic such rich grounds of debate.
If a drug makes you happy, and a film makes you sad, what makes the drug functional and the film not?