I found this very interesting, having discussed similar things with
my sister, who is a musicologist (myself having studied linguistics
for some time).  Let me add a personal remark which may highlight this:
Our little son Benjamin, our third child, who is now around 15 months
old, is our "music tester".  The moment he could sit, he started to
dance with his body whenever he heard music (on the piano, on TV,
played by his brother or sister on the recorder, in the supermarket,
...).  So we now use him to check whether something is "music".

According to him, rock music _is_ music (contrary to what Craig
Brougher sometimes seems to imply ;-) ), but he is not convinced about
rap, so it doesn't seem to be rhythm alone...

As to the larger question, even though (I said that already) I found
your text very interesting, Nathan, I must say that I have settled for
a maybe trivial answer: People enjoy music.  Period!

Regards

Harald M. Mueller
Grafing b. Muenchen - Germany
http://www.haraldmmueller.de