It can never really look or behave natively on Gnome since client-side decorations are more than a boarder and window buttons. It's also about having an integrated toolbar and more draggable area.
Don't you have this completely backwards? If I want my application to have special custom decorations, then it makes sense for my application to draw the decorations. But if I want my application to have the standard decorations of the system, why should I want my application draw them?
GNOME supporting SSDs would not diminish the ability of GNOME's applications to use CSDs. So objections about the utility of CSDs imply a false dichotomy.