The XFce root menu doesnt work with the pinboard, and 'Pass all backdrop mouse clicks to window manager' doesn't fix it.
XFce's root menu is part of the xfdesktop module, which creates a single large root window to set background image and create the root menu. ROX's pinboard also creates a large root window for desktop icons. These two compete.
However you can kill the xfdesktop and enable the ROX pinboard. When you do so the Xfce root menu will no longer work, but you will have desktop icons and the ROX-Filer menu by right clicking.
The Oroborus project has a Gtk+ 1.x application (on which xfdesktop is based) which doesn't create a large root window. It's called deskmenu, and provides a nice root menu without conflicting with the ROX pinboard.