I've been following Wayland for a while and this kind of thing has come up several times on the mailing lists. As one person who responded to Martin's post mentioned, what about consistency in the UI for the applications themselves? Why the concern about decorations?
Imo there is a bit too much concern about CSD. Toolkits might want to use a common library to ensure that decorations look the same but client side gives more flexibility afaics. Some applications might want unique looking decorations, developers can appreciate the ability to innovate in that area. If not they can just pick an appropriate toolkit that does what they want.