Because your cost calculations weighted with equipment costs and wattage results in a better earning and shorter ROI on 7950s
I'd like some numbers on that better kh/J claim. Shorter ROI doesn't mean shit if you're planning to mine in the long run.
Enjoy spending an extra $100 per GPU and extra money for electricity costs to get almost no increase in hash rate (if you can even get 700Kh/s, that is).
I'll mine with my 4 GPUs doing 700Kh/s each while you mine with your equally expensive 3 GPUs doing 750Kh/s each.
Enjoy trying to sell your 7950s when I dump my 7970s at the end of the road..