Nibali got back to Quintana easily. He should just sit in his wheel and tell him to ride till the last 2km or so. After all Quintana is in the pink. But of course now he is riding.
I think they're all just exhausted at this point. It's easy for us to sit here and say "why aren't they going?" etc, but it's obviously not that easy, they all know they need time on TD to win. Ok...here it comes with Nibs and NQ
I don't think it is in Nibali's interest. He's not good enough to take significant time on Quintana. He's better off just sitting in his wheel conserving energy and waiting till the final few kilometers.