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In fact, no theming is even necessary. Qt treats GTK/Gnome similar to how it treats OS X and Windows these days: it assumes the widget styles of the system. To use a Qt/KDE theme on a Gnome desktop, you actually have to jump through hoops, which I think is a very good thing.
Whether GTK is or isn't inferior, there is no denying that it is faster to take advantage of native Linux features.