The interesting thing is that the Grandparent post has a point. But not the one that was inteneded.
Yes, professionals are more interested in results than freedom. But, a large percentage of animation houses now use cinepaint (a fork of the GIMP) not so much because of cost but because they could rip it apart and make it do the job THE ANIMATORS need done not the job the software vender wants to do. (This is in a way what RMS really wanted out of software, not the $0, but to rip the software apart and make it do the job at hand.)
When you compare GIMP vs. Photoshop, did you look at it's integration with maya, renderman, and a host of custom animation apps? Probably not, I didn't, but making a spinning globe from a map is much easier with GIMP than Photoshop.
Some issues that Cinepaint/GIMP have in competing with Photoshop are - lack of equivilent to flight check (one printing error is all it takes to cover the $500 cost.); Lack of integration with Illustrator, (although scribus may come into the fold); the photo retouching tools in the recent versions of photoshop make photoshop 7 and earlier downright painful in comparison. (and I would say that the GIMP is probably some where around photoshop 6. A great piece of software, but not Photoshope CS or later); lack of equivilant fonts to what Adobe throws in with Photoshop. I am sure that I am missing some, but It would be great to be able to pop from film mode to print mode and print mode would be CYMK plus arbitary spot colors. film mode would be 16 bit/per channel color.
This may just be a matter of time for someone else to find that they need to extend photoshop in a way that justifies extending the gimp/cinepaint instead of extending photoshop.