Emergency cross-trainers are runners who cross-train only when an injury or pain prevents them from running. When healthy and pain-free they get all of their cardiovascular conditioning from running.
One downside of this approach is that when cross-training is treated as a last resort, runners are more reluctant to rely on it than they should be. Emergency cross-trainers often have little faith in cross-training and are thus tempted to try to “run through” injuries—a choice that seldom ends well.
Emergency cross-training can be a good approach for runners who are seldom injured, however. Nothing improves running fitness like running, after all!