Hi,
i think one of the main goals of the BackBox team was to create a very lightweight yet powerful distro. So they made a selection of the tools to be included, thus having possibly no "duplicate" tools, but certainly a big performances speed-up was given by the choice of the desktop environment. i think the developers will keep xfce (correct me if i'm wrong), a very stable DE that doesn't require much resources. Anyways, in most cases, a black terminal is enough to make the job

However if you want another desktop environment, for example KDE, you could always install it regularly, as in any other distro, and then you should be able to select the session you want to start (let's say xfce or kde) in the login screen.
Bye... have fun!