I for one cannot possibly use FTP, NFS, or AFS in all the places where I'd
use a local file. I have control over the mail system, not all these other
things. A site may choose not to have any or all of these things available.
Local files, however, are always available. In my case local files are
visible to about 40 different machines. FTP and NFS are available only
on a subset of these systems.
A local file reference is also the only feasible method to use internal
to a mail system, which is the only application of this I'm currently
in need of.