Well, I was expecting something like this after the bubbling about c/p in GATA. (I was planning on making a thread myself, but I think your worded it better than I would've)
I like to separate copy pasting into three different tiers. 1 is the most basic: taking a random image and mirroring it without any editing. Good for self, but IMO these textures should not be sold for too much. 2 is done with editing, doing minor changes to lines, lining objects up correctly, etc. I'd call that the cutoff for being acceptable. Of course, I think there should be more edit than paste. Then there is heavy manipulation, the hardest level. So pretty much just copy pasting to directly base shading off of. Ideally, the artist should smoothen out all the grain that is in most stock images, make the saturation fit that of toribash, take care of all seams, even the top, tweak shading to fit the head, etc. Hell, even make it look hand drawn. By this point you are doing enough to actually learn from the work. Hopefully, you'll learn enough to be able to draw the shades from memory, create anything you want, and be free of needing a source.
I really wouldn't mind c/p at all if it was all the third way, cuz then, they will be able to improve each time, and that is what GATA is all about: improving your techniques.
Also, I sometimes see textures with half cp, half self drawn. The problem with these are that unless your shading is up to par with reality, the c/p stands out too much and makes the hand drawn look too bad. While you may try to do this with the intention of weaning yourself off of c/p,it'd be more effective and look better to just go cold turkey. You CAN do it!