Sounds cool until the reality of price of the motherboard vs how many cards you can use with it and trying to power 7 cards. If you use this type of MB you need risers, a custom case solution and 2 large power supplies (~1000W) and some custom wiring to make it all work.  I'd estimate the per GPU cost of this solution to be $80 - $120/GPU. If you build with 3  GPU/MB you can use a much cheaper MB, a standard case, a large power supply and USB key.  If you count PS/Case/USB key that gives you overhead of  $75-$100/GPU. For me the ability to plug and play and lower cost make the decision easy.