Exactly well said, not to mention the 2 other spare GTX690's won't be doing anything besides drawing power in gaming situations, unsure if professional 3d work will take use of the extra card's, but you might be able to set it to physx dedicated mode or apps such as Folding will only be able to take advantage of the extra GPU's. As of now, there is no such thing as 6/8/12 way SLI, even 4 way SLI setups won't provide you enough of a big boast for it's value and very little games support it, only benchmarks will take use of the extra GPU's or other complex computational application that's been programmed to handle the extra hardware.
Yes, indeed. That particular PC NCIX built was for some sort of folding application and they stated that the last two card would be wasted if it was used for gaming. I was just using it as an example to support my point.