Excessive cardio will eat muscle tissue. Excessive cardio without recovery intervals will lead to overtraining and speed up lean tissue loss.
So what's "excessive cardio?" This depends on your genetics, your level of training and prior adaptation, you diet, the intensity of your runs, the duration of your runs, the frequency of your runs, the duration of the schedule, etc.
Unless your a top level competitive runner at a training peak, running 8 miles twice a day 7 days a week is excessive. On the other hand, two 10 minute light joggs a day every day is not going to kill you. The best approach is to periodize your cardio training, utilizing various training volume, intensity, duration, and of course making sure you get ample recovery time.
BTW, I think the blanket advice "don't run" is pure crap. There are without a doubt times in a competitive bb's training where he or she should not run. However,for the rest of us, in the name of good health, some form of vigourous cardio (besides resistance training) should be practiced. Now, "don't run as your sole means of altering bodyfat composition" is a statment I can live with.