Lots of different GPU's out there with a lot of different drivers and freaky weird ways of pushing things around to get the best results on one or two tasks at a time.
GPU's are great at doing a handful of things really really well.
CPU's are great at doing a lot of different things pretty well.
Once OpenCL gets proper support from the major manufacturers, I'm quite certain that there will be either an external renderer that utilizes GPUs that supports Blender or perhaps Blender Internal Renderer will get an upgrade. So far the lack of unified programming language for the GPUs has been missing, but OpenCL should remedy that in the near future.
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GPU's are great at doing a handful of things really really well.
CPU's are great at doing a lot of different things pretty well.