For me, demos are a way of sharing what I am doing right now without having to wait for a performance that may well never come. They are also very useful for myself so I can check how the work is sounding, and this is really a standard right now: I don't know of a single composer that does not produce demos beforehand.
For me, the problem with creating music with a computer but using regular sounds (violin, piano, etc.) is an aesthetic one: the computer is simply trying to imitate something, and without a very strong concept I think these compositions almost always turn to be very poor or not interesting: they are not as good as real life performance, nor they are as interesting as an electronic work that really embraces the electroacoustic world.
And you find Nancarrow hilarious? I don' think that is actually his intention, I am pretty sure he meant those compositions in the most serious way. They have some amazingly precise accelerandos e ritenutos, as well as other aspects of music, and so it's not only the sheer amount of notes that makes it impossible to be played by a human being.
Still on this subject, these days I bought a CD of Xenakis keyboard works performed by a MIDI piano (a real piano which can play itself according to a MIDI track, so a modern version of Nancarrow's instrument). The programmer (or should I say pianist) made the rhythms as precise as possible, something that no human is capable of doing either. The result is really nice and gives a very nice light to these works that I adore: https://neos-music.com/?language=english&page=output.php%3Ftemplate%3Denglish-album-details.php%26content%3DAlben/10707.php
PS: regardless of anything, Alkan is still unplayable