The free software movement is a political cause, not a technical one.

Richard M Stallman writes in emac-devel list.

  What I'm trying to say is: I won't discuss which dVCS we choose
(unless it makes Windows development a PITA). But I agree with Jeremy
Maitin-Shepard that the cause of free software is strengthened by us
selecting among the free alternatives the one that best serves our
technical, not political, needs.

That is completely backwards.  The free software movement is a
political cause, not a technical one.  "Choose based on technical
criteria first of all" is the opposite of what we say.

There are many reasons why GNU packages should support other GNU
packages.

The GNU Project is not just a collection of software packages.  Its
intended result is a coherent operating system.  It is particularly
important therefore that GNU packages should work well with other GNU
packages.  For instance, we would like Emacs to work well with git or
mercurial, but we especially want it to work well with Bzr.

The maintainers of one GNU package should use other GNU packages so
they will notice whether the packages work well together, and make
them work well together.

We also promote use of other GNU packages in this way.
Other people don't necessarily see which editor you use,
but they all see what dVCS you use.