For distributions that use policykit, xfce4-session needs to request the necessary permissions from policykit to shutdown when there are multiple users logged in. There are PK helper functions to do this, xfce4-session just needs to run them. 

Otherwise you get a dialogue when you attempt to shutdown/restart and are return to the GDM login screen after dismissing it:

"Unable to perform shutdown

org.freedesktop.hal.power_management.reboot_multiple_sessions
auth_admin <-- (action, result)"

Cheers! :)