It isn't too bad as long as the light sources of the 2 images match really well. I think duesentrieb is right that people get mad because they beat their brains out trying to get the poser doll face to look more real, and they see pasting a face on as a dishonest shortcut. I'm not against it in principle. It depends on the results, I guess. I mostly stick to using 2D for backgrounds because my Bryce skills are merely adequate.