Created attachment 288425 [details]
Screenshot showing docked nautilus on the left over a maximised nautilus in the background

Windows docked to the left or right lose their window borders and this makes it impossible to tell them apart from windows that may be maximised in the background. If anything, it seems to me, borders for docked windows are all the more important that other "floating" windows to give the user some feedback on where the window's extent ends, e.g. while writing something in a gedit window docked to the left.

A screenshot probably explains better: the attached one shows a docked nautilus on the left over a maximised nautilus (inside a blank folder) behind it. No telling where the docked nautilus ends and where the maximised nautilus in the background begins in terms of screen estate.