For me it's just an ease of use issue, but I can think of people mounting
their media via nfs/cifs or sharing them between sites & forcing URL_WWW
and (especially) PATH_SERVER to be MEDIA_URL and MEDIA_ROOT relative would
certainly make their lifes harder or at least make their
code/filesystem layout look suspiciously like spaghetti.
Also, I don't really see a point in sacrificing flexibility for ease of
use. No matter how idiot-proof you make your software, sooner or later the
Universe will come up with better, bigger idiots ;)
Regards,
GrF