My rear brake cable has been fraying for a long time, and instead of replacing it I just tightened up the brakes to keep it in place. About a year ago I was riding to work on my bike and pulled up to a four way stop, hit the brakes like usual, and felt a snap. The brake cable had, of course, tore apart where it was frayed, and if it weren't for my front brake I might have found myself in the middle of the intersection.