Originally Posted by Calsun

Pro racers often exceed 50 mph on downhill runs and do worry about heating the rim with standard brakes which have been little changed over the past 100 years. With disc rotors that is no longer a concern. With a good rear disc brake there is also less need for a front brake and these may disappear on bike built for time trials.

Pro racers need to win and if that means Shimano disc brakes instead of Shimano V-brakes that is what they will insist on using. It does not matter so much about the sponsor and often racers have used different bits of gear carefully disguised so as not to show up in the photos. Even the frames may have a given brand name but are hand built to the rider's specifications and this has been going on for as long as there has been professional bike racing.