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The toolkit doens't have anything to do with the dropshadows, that's done by the windowing system. If you look at the screenshots, the GTK windows do have dropshadows: http://people.imendio.com/mitch/screenshots/gimp-on-os-x.png
As for "more graphically sophisticated", I'm not sure what that means, but Cairo gives you features comparable to Quartz.
So isn't this a step BACKWARD for that platform?
You don't have to use it, but GTK opens the door to making more native-looking ports of Free Software. For whatever reason, there doesn't seem to be a lot of native Free Software.