"A bloke I know ......
When I was chatting to the owner of road bike with cable discs last week, he said the performance, especially in the wet, was far superior to side pull brakes.
This reflects my own experience with the hydraulic discs I have had on my MTB for 5+ years, they are certainly much better than the cantilever and vee brakes on my previous/other bikes. Also, despite my initial concerns about maintenance, they have been virtually maintenance and problem free. (Perhaps I should have done more maintenance on them, change fluid? but I haven't and they still work fine
) I must confess though that I don't do much MTB stuff now.
On the issue of weight on road bikes, he was apparently told by one manufacturer that more widespread discs on road bikes was only a matter of time, the overall weight change can be made neutral, because disc brakes will be made lighter and as already mentioned, lighter rims can be used. We'll have to wait and see!
As an engineer they seem to be a logical improvement, eliminating rim wear seems to be a worthwhile bonus and I agree with one of the previous posts that a cable disc brake isn't terribly complicated. (However, given my luck with wrecking wheels, I'd love to own a set long enough to wear them out
)