Earlier quoted context omitted.
Being able to use Gimp's feature set in the 3d view while texture editing would be mindblowing. Or even just seeing changes live on the model.
I wouldn't be surprised if that comes someday, since there's already super basic painting on UVs, but "real" work tends to require exporting and working in some form of 2d app be it Gimp or Krita or whatever. Having the same level of functionality directly in the UV editor or even directly on the model to help deal with seams would make shit SO MUCH easier.
Also, the way UV editing selection works is rough. If you don't have a face selected in the model, it simply does not show up in the UV. The UV has its own selection. So in order to single out a part of the UV based on a set of faces you need to first select the faces, remember where in the UV map they live, then select the whole model again, then select the UV points again.