What is the reasoning behind :background zenburn-bg?

[angrybacon]

For many faces (other than default), the background is set to zenburn-bg. This makes it pretty verbose to update those faces them when customizing the default background.

Another option I currently use is the following:

(zenburn-with-color-variables
  (mapc
   (lambda (face)
     (when (eq (face-attribute face :background) zenburn-bg)
       (set-face-attribute face nil :background 'unspecified)))
   (face-list)))

Which obviously doesn't work unless run after all the packages are loaded.