Not on FreeBSD but on OpenBSD:
xfce does not always gets the sudoaers right (greyed shutdown and restart).
However, once sudo has been used on any other app (even CLI), the functions are available.
On FreeBSD, with both GNOME and xfce, the GNOME sudoers functions are available, at the cost of not beeing available to xfce.
When on FreeBSD, GNOME is not installed (sort of as about everything gets installed for xfce "deps"), xfce has access to the sudoers functions.
Must have to do with some other security thinghies a la policykit, fam, gamin, ...
won't keep me awake at nights.