Road Bike Disc Brakes
People have short memories. Disc brakes are the norm on mountain bikes not because they are better at stopping you - they are not. A well adjusted cantilever V brake will throw even the heaviest rider over the bars. They are, however, a useful tool for those venturing off road in poor conditions and those who take pleasure in distorting their wheels.
Cross country mountain biking in wet weather eats brake blocks - the v brake compound creates the friction to stop you by disintegrating in wet conditions - you can start a day with nice new brake blocks and finish with none. Grit accelerates this process and acts as sand paper on the rims. V brakes can clog up with mud and even a mild wheel buckle can necessitate a trail side repair.
With disc brakes riding off road in inclement weather is not a problem. The block compound is much harder so wear is minimal, mud is less intrusive into the brake calipers and if you buckle the wheel the chances are it will still rotate in the frame and get you home with operational brakes. All things considered if a mountain bike is used for it’s intended purpose the disc brakes are worth the extra weight.
Fast forward - road racing. Road racing is tactical and saving energy is key. Training involves spending countless hours on a bike to get as light and efficient as you possibly can, to do as little as you possibly can in competition. In road races, time trials and triathlons, speed is key and for every watt to count as much speed as possible is carried through every corner. Braking is kept to a minimum.
To facilitate speed, racing bikes are as light and aerodynamic as possible. (Block wear is not an issue and most wheels don’t survive the 50,000+ km it takes to wear through the rim). Disc brakes add weight (about 400 grammes per disc) and a nice carbon deep section wheels aerodynamic properties are reduced to that of an old fashioned 32 spoke box section alloy. So why?
The only people set to benefit from disc brakes on road racing bikes are the manufacturers - they would love the UCI to allow disc brakes so that they can inflict them on the professional peloton. Contrary to common belief professional riders ride what they are given and disc brakes in the pro peloton will make them the next must have item for the hoi poloi. Expensive and durable carbon frames, wheels and shift levers the world over will need replacing - ker-ching.