I don't have a strong opinion on the question of CSD. I can certainly see the benefits it offers, but also the problems.
> And Chromium
Chromium on Linux has an option "Use System Title Bars and Borders" which turns WM decorations back on. (Click on the empty area next to the tab bar)
> I think Firefox does it now too.
Not on my system at least -- still looks Gnomey.
> X11 offers (and forces you to use) myriad ways to tell the window manager to shut up and get out of the way
That's not a problem really -- most apps will accept WM decorations for their main window because that's the natural thing to do. The ones that you're listing (Chromium and XMMS) are exceptions.
The problem is the *common* case with Qt apps running on a Gnome desktop or GTK apps running on a KDE desktop -- they will look inconsistent and possibly behave inconsistently too.
Or consider toolkits that until now didn't *need* to render their decorations -- I wonder whether Tkinter and FLTK even have any decoration-rendering code because usually the OS will be happy to provide that. Now they need to implement decorations themself or rely on some new external dependency (probably a competing toolkit).