I've been using GiMP and Inkscape for years professionally.
The only issue I've had with GiMP is that it saves smaller web sized images at a larger size with less clarity than photoshop (typically happens with smaller images used for buttons and such, even when you add the "save for web" plugin to it), however for anything larger it's been fine. Though I use Inkscape/Illustrator for web design now anyway and just use GiMP/Photoshop to make the few things I may need it for, so that doesn't cause much of an issue for me anymore.
Inkscape can definitely work as an illustrator alternative, just save your work as svg(the default), and it can even be opened with illustrator.
I don't think the problem with linux graphics programs is that they're not as good, in fact in most cases they are better. The problem is that the programs haven't added new features in a long time, and there seems to be no effort to make GiMP, Inkscape, and Scribus better and make them work together as a suite. Aside from that, they would need to be able to import/work with adobe formats as well. (which GiMP does very well with photoshop files, you can even install photoshop brushes in it.)
If that would happen, then making then common in the work place would not be much of a stretch.
*edit: if anyone has issues using gimp, but not photoshop, try remapping all of the gimp shortcuts to the photoshop ones. Once I did that GiMP became a breeze to use.