Except that the "desktop metaphor" stops being a "desktop" with the introduction of a cursor and mouse. Actual desktops are manipulated directly by users... and desktops don't allow you to move your hand all the way to the right and magically move to a new clean desk (in the case of virtual windows). There's nothing about a desktop that infers the behaviors described which is why it's crappy for conveying the actual behaviors of current UI... hence the limited nature of metaphor.