As with many things, I'd say it depends. Variety in workouts can be beneficial for both strength and stamina. I have three issues, though, with saying that running helps. First, I don't feel running provides the change in a workout (from cycling) you can get from other activities such as rowing, swimming, or racquetball. Second, if you're talking about adding an activity, running can have detrimental effects on appetite and create an overuse impact on muscles. Third, if you're suggesting replacing some cycling time with running, the potential is obvious. My preference is rowing to supplement my riding because I can get a good cardio and overall workout without stressing my biking muscles and joints.