The riding I do on the mountain would be very hard to accomplish without the power and modulation that comes from disk brakes. All the mountain riders I know are excited about road disks. Seems also disks solve a lot of problems associated with carbon rims, and would allow on high performance profiles for the aero disadvantage to be overcome by not needing to design in a brake track.
This hasn't been talked about much, perhaps the only hinting at it I've seen from the industry came from Damon Rinard, but I have very serious doubts that moving the brake track from the rim is going to do anything substantial for aerodynamics. We've reached a bit of a plateau in terms of rim design, where the current crop of fastest wheels (all exceptionally close) don't have parallel brake tracks, Mavic has their little filler covers, Bontrager has tires with little wings to smooth out the profile, etc. I think we're beginning to see a flattening of aerodynamic gains, everything is starting to look very similar in the grand scheme of things.
From Damon when people were asking about the P5 testing: "We just got back from a tunnel trip and it's not looking pretty for disc brakes... I can't reveal the numbers yet but I'll just say that adding a front disc brake and caliper to any "superbike" would add enough drag to make it slower than some road bikes... "
If true, that's an enormous amount of drag from just a front caliper. I'm watching the development, but I'm skeptical at this point.