Nobody is riding discs in the mountain stages because the extra weight would represent an insurmountable performance penalty. Enough of a penalty such that the sprinters might not make the time cut-off.

They were used by a couple of riders on the pancake-flat stages as a marketing gimmick. Kittel is such in overwhelmingly dominant form right now, that he would have won these stages on a Pee Wee Herman cruiser bike.

Discs also represent a major liability in crits due to the constant accelerations. I was watching our local Pro/Cat 1 race last night. Out of over 100 guy entrants, a single rider had discs, and he was shelled off the back real fast. None of the ladies had discs.