Gimp can do most of the stuff PS can do and is *free*. A lot of PS pirates are just too lazy to get past Gimp's learning curve, same as with Blender and Max.
Blender, though, is catching up a lot recently, see
http://www.polycount.com/forum/showthread.php?t=72805
I bet the same will happen with Gimp.
2.8 got tablet support and single-window mode already. And there are artist packages like Gimp Paint Studio.
Plus, there is Mypaint which is basically a tablet friendly drawing front end to Gimp. And it is painless to move images between the two, including layers, with Open Raster format.
It really is good enough for most of the people who think they have to pirate PS. Those who really *need* PS probably don't need to pirate it...
In the end, it's not so much about tool X or tool Y, it's about skill and I dare say you can develop those skills just as well in free software.
You cannot pirate skills.