The xfce4-session-logout.desktop file seems unnecessary when, especially when it turns up when searching in the whisker menu, as session closing buttons are available in the menu. Either the .desktop file should be removed or alternatively it can be hidden from the menu by adding the 'NoDisplay=true' key to it.
Seems the file is needed by the default non-searchable application menu, so ideally we should try to hide it from appearing in the whisker menu.
I don't see this as a problem. The application finder also shows this launcher. Additionally, users are free to remove all of the session closing buttons from Whisker Menu.