The problem now is people conflating Canonical's own design decisions (most of which I don't agree with, except for the case of netbook UIs, but that's not the point here) and Client-Side Window Decorations support.
Martin Gräßlin's blog lists a few things that you would lose if we were to use client-side window decorations. Most of them are just wrong:
- Consistent behavior between all applications no matter if it is a Qt or a GTK or $Toolkit application: How can you say that when there's no agreements on the implementation yet? Of course Athena widget apps will look different, they already do. As long as the theming and behaviour is known and agreed upon, there's no reasons why it should happen. It's just a bit more complicated because you would have cases where the Window Manager would behave differently from the toolkit. All those are solvable, and old, unmaintained toolkits will not integrate.
- Window Tabbing (KWin specific): Why would that be impossible to implement? You'd just need help from the toolkit to do that.
- Window rules like always show a close button even if the window is not closeable: Working around broken apps? Fix your apps...
- Accessibility features like big border and button sizes for all windows: Certainly not. It would even mean that you wouldn't get a disconnect between application and window manager implementing accessibility features.
- Easily changeable window themes: Why wouldn't they be easily changeable? That's highly dependent on how the theming is implemented in toolkits. I guess it would be the case if you had a half-hearted implementation.
- Shadows which are part of the theme (KWin would not paint shadows for a client-side window-decorated window): Why not? If KWin knows that the application is drawing its own decorations, it could draw the shadows, or you could make the application's toolkit be aware that it needs to draw the shadows. Either way, it's not impossible to implement.
There also doesn't seem to be a list of thing you'd end up winning:
- Tear-free window resizing (when the client is doing the resizing, with a proper resize grip for example)
- Better integration of resizing within applications (say "zooming" when going to fullscreen
- Proper way to do tabs in titlebar, a-la Google Chrome
- Window-as-a-document/object (see the tab interaction part of this Empathy UI review, which would enhance the integration between applications and file managers)
And that's just the things I see myself as winning. There's more technical details on the GNOME Wiki.
Update: Got my knickers in a twist over Client-Side Windows vs. Client-Side Window Decorations, fixed up links and text.