The real advantage of it is that it allows installation of hosted engine easily, and a 'single pane' overview of a host (oVirt Node status without using nodectl, etc)
Since Cockpit wants to deal with dbus, the push is for other utilities (postfix, snmpd, etc) to publish relevant information over dbus, rather than teaching Cockpit the format of each and every configuration file on the system.
I wrote the Tuned plugin for Cockpit, and I maintain the oVirt plugin. I gave a talk at Devconf about plugin development, which is extremely easy, but the Cockpit team is small. It's very easy to write plugins for it, but even easier if they expose dbus.
That said, the cockpit API (http://cockpit-project.org/guide/latest/api-base1) makes handling files/processes from client-side Javascript in plugins simple. It's just that you'd need yet another config file parser...