a) without a standard way of loading a print server's disks with
any data, mandating support for the "file:" scheme would hardly
lead to interoperability.
b) While one can imagine "if you support remote printing at all,
you should be able to retrieve files from an HTTP server", since
it just requires a network connection, it doesn't make as much
sense to mandate a "file:" scheme because it presumes that the
print server has a file system with a naming system that is URL
accessible.
c) the "file:" URL scheme is in serious need of attention: the
implementations (even on the same platform) are widely divergent,
and not conformant with the recommended practice in the RFCs.
It's not that this is really a terrible idea, it's just not clear
that it's useful to "mandate" something that by most measures
has to be optional.
Larry
-- http://www.parc.xerox.com/masinter