Comment 1 for bug 573168

> Given that we have a "base", "theirs" and "mine", it seems a simple change to add an "M: attempt to merge" option.

The problem is that your Ubuntu (or Debian) system does not really have a "base" version of that conffile, only a checksum stored with the rest of the package information in /var/lib/dpkg/status.

That fact would either have to change first or you'd need the original package, which is usually available.

Storing each actual conffile might be prohibitively large amounts of space if you have bad packages.
That would be a decision that either needs to be taken by someone with an overview over these things or it has to be opt-in on a per system or per-package basis.