At the moment debcommit -r provides commit message only when creating signed tags:

    if ($signtags) {
        if (defined $keyid) {
            if (! action($prog, "tag", "-u", $keyid, "-m",
                         "tagging version $tag", $tag)) {
                    die "debcommit: failed tagging with $tag\n";
            }
        }
        else {
            if (! action($prog, "tag", "-s", "-m",
                         "tagging version $tag", $tag)) {
                    die "debcommit: failed tagging with $tag\n";
            }
        }
    }
    elsif (! action($prog, "tag", $tag)) {
            die "debcommit: failed tagging with $tag\n";
        }
    }

but actually with git it should always provide -m  to create an annotated tag,
which are the "real" tags in GIT.  'git tag' without message/annotation creates
merely a pointer, without any meta-information (e.g. date/author,etc) and thus
such tags do not even considered by 'git describe' by default (explicit --tags
is needed).  such lightweight tags could be used by people to mark some
temporary or non-release specific locations in the development history,
so just demanding using --tags all the time is also suboptimal.

more of food for thought on why to use annotated tags:
http://stackoverflow.com/questions/4971746/why-should-i-care-about-lightweight-vs-annotated-tags

Cheers,