The reason I have to ask it is the integration with designing-specific features GIMP should also provide.

If I am working on a website, I might want the windows to look someway that they give me the website layout, for example. Or if I'm doing some print design for this tshirt, I may want to have displayed somehow that the image I am working on shows on different tshirts models I have... 

I like the way it works on MX and CS3. And I'm not the only one to think that way sine Adobe changed from "Floating windows" like GIMP to a more MX MDI style.