The concern isn't just that the vulnerability surface has increased: it's that we can't even easily tell what it is anymore.
Determining the set of privileged code that could carry out operations on behalf of unprivileged users was fairly simple in the days before PolicyKit: find setuid/setgid binaries, chase their shared library dependencies and (if you're paranoid) see what they can dlopen(). Just a grep away, in any case.
Now, we have to analyze the dbus and PolicyKit policies as well, and XML is... not terribly amenable to analysis with Unix-style shell tools. (Some Perl packages come with XML-style XPath-based grep tools, but they are a) rarely installed and b) seriously cumbersome. We really need an awk for XML.)