If you wanted to race it you couldn't. The UCI requires that the bike weigh at least 15 pound to be legal for racing. Man those are some pretty nice rides. What you think the reliability of a sub ten bike would be. Unless I was climbing a 10% grade and did not have to come down the other side I don't think I would have a lot of confidence in a 8 pound bike but then again I'm not 140lbs.
I bet the bike would climb good. But yeah, I don't think I would be using it. I think it might flex a bit too. Check out this 14lb dual suspension mt bike (i think that is pretty amazing too). It has a plush seat for metro to sit on /