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Disc brakes are preferred if you have carbon wheels. You need special pads for braking on a carbon surface and carbon fiber is much less resistant to abrasion from the braking pads. The net result is that you need to replace your expensive carbon fiber wheels much more quickly and your braking performance is much worse because brake pads for carbon fiber don't let you stop as quickly as regular brake pads. Much better to have the wear and tear from braking occur on a $20 rotor that you can easily replace.