I'm a fellow Xfce user under Slackware 14.1.
IMHO, I find the Slackware's Xfce experience more "complete" with what's provided under the /xfce dir than what Debian actually provides with their Xfce "install task".
As someone else already stated, most of the remaining Xfce "goodies" are available in SBo.
I'm considering switching to Mate mainly because of the increasing number of GTK3-based programs.
However, I might give the Xfce team some help in their GTK3 migration.
I'm motivated by the fact that Xfce (and probably Mate too) is a sane DE that should keep going.
Of course the above statements are only from my humble opinion.