So I think I agree with you that debian/rules is only used for distribution. In our case its only our graphviz packages that we distribute, but we have to deal with the variation between the various ubuntu releases.
In my mind it is the equivalent of the spec file for rpms, which similarly have to deal with variations between the rhel, centos, fedora and multiple different releases of each.
I don’t understand your comment about testing. Aren’t the builds from graphviz master branch that are considered deployable to the multiple distros, aren’t these the most important place for regression testing?