But docks work in a different way, such as XRender or just a PNG with an alpha layer. Much less expensive than copying a chunk of the backdrop and then "pasted" at your terminal background.
Often docks will be able to show you the underlying windows behind it, even without compositing. But under the terminal with fake transparency you couldn't see another window behind it, just the backdrop part.
I think X11 had some extensions to allow translucency too for software like oneko, xroach or such.
From my experience this is wrong. If you set a transparent background in wxwidgets/gtk it will show you a grey background. And gtk3 even removed the ability to query the x background pixmap. So if there is better support for this at an X level, this is not available above.
