At least xfconfd always stays in memory after you log out of Xfce session. Probably some other stuff too, I'm not sure, but I can certainly say it about xfconfd. It causes some problems too, as you can see here: https://bbs.archlinux.org/viewtopic.php?id=203080 and here: https://forum.xfce.org/viewtopic.php?id=11490
Another issue: it gets very hard to copy/edit configuration files if xfconfd is kept running. Usually, logging out was enough. Right now one also has to kill xfconfd after logging out of X, otherwise the new/changed configuration is overwritten when logging in again. See https://unix.stackexchange.com/questions/381276/xfce-panel-config-file-is-rewritten-with-old-contents/411999#411999
*** This bug has been marked as a duplicate of bug 15368 ***