If the Exec tag in the .desktop files under /usr/share/applications contains an invocation parameter and a special parameter like "%U", the invocation parameter gets lost while transferring the command the the menu cache. eg: the following .desktop entry: Exec=ooffice -calc %U becomes: cmd="ooffice" in the menu cache. Without the "%U" everything works fine and you get: cmd="ooffice -calc".
libxfce4menu has not been part of Xfce 4.4. I'll move this bug to xfdesktop which handles the menu in Xfce 4.4. Anyway, all I can say to the complaint itself is this: in the menu, you can't pass any special arguments to programs. Thunar for instance has "Exec=Thunar %F" but when you click on the menu item for Thunar, you don't want it to open a special directory (maybe HOME, but that should be handled in Thunar). So what almost *every* menu implementation will do is removing those special arguments. BTW, in libxfce4menu the original Exec= values will be kept so that all clients (like xfdesktop or Thunar) have to handle (in case of the panel menu: remove) them on their own.
This is fixed in both trunk and 4.4 branch; should be a part of xfce 4.4.2. *** This bug has been marked as a duplicate of bug 3106 ***