Its hard enough to push an out and out xc bike to it's limits short of Fort Bill or Inners
I'm not saying it would be fun or as fast and obviously an 80mm hardtail is going to take a biker of bitchin' skill to ride it down there but the bike would be fine. I'm just confused as to why such extreme bikes are necessary and furthermore considered all mountain when they are really not an all rounder[/i]. In my humble opinion the orange 5 and its peers are all mountain as can be seen from their capabilities. There might be a dozen world cup DH courses where they wouldn't cope but for everything else they would be fine.
Taking rider skill out of the equation, and assuming by "extreme" you mean 160mm travel bikes I'll tell you why they are considered all mountain or all rounders – compromise. They will climb a hill as well as can be expected and then get down the fun stuff as fast as you can short of a DH bike. You might not be willing compromise climbing ability for descending ability or durability but others happily will.
And whilst an 80mm travel bike can be rattled down most things you absolutely will not be faster than someone on a Reign X or similar nor will an 80mm travel bike with xc components last very long if used on Fort Bill for example, day in day out. You might feel faster but I promise you you aint. You'll be quicker up, sure, but some people want to push the limits of their own skills and bravery on the any terrain they come across, not just the limits of their xc bikes build quality and they are willing to lug around some extra heft to do so.