While it may not be as important to the weekend warrior, disc brakes can pretty much assure you that you wont be on foot. There are many reports of cross country riders being on foot when their rim brakes have destroyed their rims. Even if one of your disc brakes fail, you still have the disc brake on the other wheel, that can keep you going.
The thing here is the fact that most cross country riders ride rain or shine. When riding in the rain the rims and brake pads pick up fine grit and accelerate rim wear even to failure. That wont happen with disc brakes. One last point too is a warped rim beause of a broken spoke or what ever dont effect disc brakes.