Such as? This sounds very fishy to me. The lightest traditional spoked wheels are around 500-600 grams for a front wheel, and 750 is easily achievable at a reasonable price ($100 or so). If you're ready to spend 3000 Euros or so, a front+back set of road racing carbon Lightweights comes in under 1000 grams. I'm absolutely certain that no 6-spoke aero wheel comes close to that. That's road wheels but MTB is pretty similar, I think.

As to the OP: for everyday use, they are just a pain. Expensive, heavy, unrepairable, sensitive to crosswinds. People buy them either for racing or for the looks.