The harm of only doing cardio when you work out is that you are not building strength in your muscles. You will not get that "tight and toned" look and you will actually decrease your muscle mass. You can also develop tight muscles reducing your level of flexibility. –Sara

The harm in only doing cardio while working out is that by doing so, you’re neglecting many other parts of fitness such as strength, and flexibility.  Many people are drawn towards cardio only workouts early on because of the potential to see quick weight loss.  However, this is generally accompanied by loss of strength and muscle mass which in the long run will slow a person's metabolism down and sabotage long term weight loss goals. -Keith