IMHO hydraulics are unnecessary on a bicycle. Sure you don't have to adjust them but if they go bad (the master cylinders seem to be the biggest point of failure) it's an expensive fix. In fact the very small volume of a bicycle's hydraulic system makes it very sensitive to water contamination and the need to make the system as light as possible makes it relatively fragile when compared to say, a motorcycle's hydraulic system. Strong braking power and good feedback are not exclusive traits to hydraulics... cable actuated discs can be just as effective if designed/matched properly and set up by a good mechanic. However that last point is one major issue with all disc braking systems for bikes. It seems that not many bike mechanics know how to set them up, maintain them or adjust them properly. I've had my fair share of having to correct issues deemed "hopeless" or "the nature of the beast" by other mechanics.
My MTB probably has the cheapest cable actuated disc brakes you can find yet I can controllably find my tire's limits of adhesion when at speed... I certainly don't think that hydraulics would serve me any better.