It does fill a need or 3, I must admit, and I hate to be negative, but if that is the best example you can come up with of the positive effects of putting Emacs functionality in external programs, that makes me lean towards the conclusion that it is a bad idea to put Emacs functionality in external programs!
mu4e: http://www.djcbsoftware.nl/code/mu/mu4e.html
not much: http://notmuchmail.org/
EDBI: https://github.com/kiwanami/emacs-edbi
ropemacs: http://rope.sourceforge.net/ropemacs.html
RSense: http://cx4a.org/software/rsense/index.html
GCCSense: http://cx4a.org/software/gccsense/index.html
emacs-ipython-notebook: http://tkf.github.io/emacs-ipython-notebook
emacs-jedi: http://tkf.github.io/emacs-jedi
request.el: http://tkf.github.io/emacs-request/
Well, the last three is my projects so you could exclude that :)
Also, we could add version conrolling interfaces such as magit and VC (relying on git/svn/hg/...) and advanced interpreter such as SLIME/nrepl.el/geiser.
Ah, and don't forget dired!