I vote for it only to be used for the headerbar/titlebar, but that’s only if we’re going to be using a dark headerbar. Are we? With a light headerbar the transparency is either distracting or so subtle that there’s no point in using it. In the case of a dark headerbar, the #181818 that’s being used is a little jarring (Unity 8’s titlebar didn’t have the issue since it was so much smaller, and yes, I know it’s darker than that in the United theme, but whatever). Today I did some testing:
I lightened the black a little (#252525 as opposed to the #323232 from your mockups) and made it a little transparent (0.96).
I find the transparency does a good job of softening the headerbar’s black without having to brighten it further (if that’s something we don’t want). I also made unfocused and maximized windows have a solid headerbar and so far it’s a decent way to distinguish between focused and unfocused windows, something that @CraigD points out is a problem with dark themes.
Transparency in the headerbar also carries over to GTK2 programs without issue and I think a consistent look between GTK3 and GTK2 apps should be something we prioritize.