I guess you have to decide if you are a GNOME app, an Ubuntu app, or an XFCE app
for the first time we may have ability to really shape the user experience and form an identity for the GNOME platform
Why would the windows manager be a "brand", rather than the distribution? (Red Hat, Ubuntu).
There is no point (for them) in people "recognising the install" if they don't have easy access to it.
Having wasted enough time configuring appearance, I now would be in one of two scenarios:
1. Install a Linux distribution such as Ubuntu or Red Hat and then use whatever comes as a default (and avoid fiddling around to save my time for more important things). Won't use Gnome if it is not the default.
2. Install whatever allows me to use the same appearance as in the other machines. SuSE/KDE used to have a "Redmond" theme that looked and behaved like Windows Classic. Do the same in the Windows machines.
GNOME is not a window manager though. The (default) WM in gnome has been sawfish, metacity, and currently mutter. Various components come and go, but the umbrella that is gnome stays, and the brand it has is pretty much the only defining attribute of it.
That's not a Gnome dev that's William Jon McCann, the main Gnome 3 designer. That McCann claims ignorance of XFCE without shame is pretty shocking.