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.
As long as the kh/J rating is the same, there are no 'extra electricity costs', relatively this stays the same but yet you reap better benefits from lower difficulties, because you have more hashing power.
You make loose claims and sound butthurt that you don't have 7970's somehow. The cards don't instantly lose their value either, so all bullshit about ROI is mere speculation as well.