If all of a menu’s commands are unavailable (dimmed) at the same time, dim the menu title. Users should still be able to open a dimmed menu to see its contents.
If all of a submenu’s commands are unavailable (dimmed) at the same time, dim the submenu title.
For most applications, if no windows are open or on screen, only the application menu, File (or equivalent), and Help should be available and displayed as standard fully saturated black text. Assuming none of their contained commands are usable in this context, the Edit, View, and Window (unless there are minimized windows) menu titles should be dimmed.
Why don’t most OS X developers implement menu title dimming? Apple’s own adherence to these guidelines is less than stellar (see iCal, iPhoto [both of which have the View menu in the wrong place], iChat, Mail, Preview, and Address Book for proof), so I’m not surprised that third-parties have not stuck to it. If there is some technical reason that it is difficult to do, it is understandable that developers would invest their resources in more obvious and/or important issues.
iTunes is one of the only applications I know of that implements top-level menu title dimming (it has no submenus to dim), which I think is unfortunate given the potential benefit to users — quickly focusing their attention on the commands that are available in the current application state.