I used to put QDockWidget inside layouts sometime : it allows, when you're not in a QMainWindow, to have a widget that can be extracted from the GUI (becomes a floating dockwidget) and put it back after.
I am aware that it is not the intended use for dock widgets, but it was sometimes quite useful.
It used to work perfectly until Qt 4.6.1, but we have now the following behavior : the dockwidget can be undocked (floating), but this cannot be reverted (the button in the title bar of the dockwidget is not visible anymore).