Xfce is built on GTK+2, the same UI toolkit that GNOME uses, so GNOME applications fit in very well on an Xfce desktop. There are quite a few useful ones that would be nice to ship by default in Xubuntu, but due to their dependencies on libgnome and libgnomeui we don't include them on the CD.
These two libraries contain APIs many of which already have equivalents in GTK+ or are no longer needed, and the long-term plan for the GNOME project is to get rid of them, see Project Ridley.
Unfortunately that project progresses slowly, as only a few APIs get into new GTK/glib releases at a time, and then it takes even more until all GNOME apps start using them.
During the past release cycles a few GNOME apps either totally lost these legacy dependencies or included a --disable-gnome ./configure option, which allowed us to build GTK-only variants of them, and they were slowly added to the Xubuntu install (gdm, gnome-system-tools, gnome-mount, evince). This is a good thing because it avoids duplication of efforts in supporting different upstreams in Ubuntu and causes less confusion for users as these are areas where having multiple choices is not of much value.
I'd like to thank Rich Burridge, Brian Cameron, David Zeuthen and Carlos Garnacho for being open to such changes and taking the patches for the projects they maintain.
Since packaging Xfce for Xubuntu was taken care of very well by Gauvain Pocentek and more recently by Lionel LeFolgoc, I mostly spent time on such patches that reduce libgnome dependencies in upstream GNOME modules. The most recent is getting gcalctool rid of libgnome, so it's a plain GTK app, should be in Gutsy soon. It will simplify the Ubuntu packaging as there's no need anymore to provide separate gcalctool and gcalctool-gtk binaries.
I'd like to replace xscreensaver and use gnome-screensaver by default in Xubuntu. Having gnome-power-manager instead of the very limited Xfce battery applet would be a great improvement too. Adding network-manager to the default install is another fine goal.
There are patches in gnome bugzilla that further these goals, I'm hoping their maintainers will have time to look at and consider them during the 2.20 cycle so I can thank them too ;)
Not to mention these changes improve the GNOME desktop as well, as they reduce the resident memory footprint of the affected apps by at least a few hundred Kbs each.
The bugzilla links: