> After more thought, I think this should remain in tools as a demo.

I disagree, I like the command line interface. It's very useful on Windows for example. It's also useful on UNIX embedded devices where Python is installed, but only a few UNIX tools.

If you are not convinced, please see this amazing talk of David Beazley at Pycon 2014:
http://pyvideo.org/video/2645/discovering-python

The patch only adds a few lines to difflib.py.

IMO difflib CLI is even more useful than tarfile CLI ;-)

It's not like no other Python module has a CLI. Modules with a CLI of Python 3.5:

aifc
base64
calendar
cgi
code
compileall
cProfile
dis
doctest
filecmp
fileinput
formatter
ftplib
getopt
gzip
imaplib
imghdr
inspect
locale
mailcap
mimetypes
modulefinder
netrc
nntplib
pdb
pickle
pickletools
platform
poplib
pprint
profile
pstats
pyclbr
py_compile
pydoc
quopri
random
runpy
shlex
site
smtpd
smtplib
sndhdr
sre_constants
symbol
symtable
sysconfig
tabnanny
tarfile
telnetlib
textwrap
timeit
tokenize
token
trace
turtle
uu
webbrowser
zipfile