The reason why you have discs on MTB's is to be able to stop effectively in the situation of wet and muddy conditions not really from high speed descends.
Conversely it's pretty rare that you find muddy conditions on the road like you do off road like the side of a mountain.
It's pretty rare that you will be doing 50-60 mph down the side of the mountain like you do on the road on a MTB.
Yes there the dare devils that find a somewhat smooth track down the side of the mountain but for the most part MTB's do not go down the side of the mountain at the same speed as a road bike on a paved road.
If you think you will get better braking performance by putting disk brakes on a road bike during normal dry conditions I would be hard pressed to see this. In wet conditions it's a different story.
So to recap disc brakes are great on mtb's in wet muddy conditions. Cantilever brakes are great also on MTB and cross bikes but not as effective as the disc brakes in wet muddy conditions.
Road bikes brake effectively with the current dual pivot brakes, good pads, and machined rims on dry roads. In wet conditions the brake pads can be changed for better braking as well.
I would not mind having disc brakes on a road bike I would use for training in wet weather though.
Aside from having good brakes it's the riders expertise in inclement weather to be able to stop and maneuver the bike in adverse conditions.
When I think of wet weather conditions on the road I think of the 89' WCC. Balls to the wall in wet conditions at 60 mph.
I wonder what kind of brake pads they had http://www.youtube.com/watch?v=yJSgzHTRg38