Principle of Specificity

"The principle of specificity explains the outcome of a given type of training program such that the exercise response is specific to the mode and intensity of training.  In other words, only the physiological systems emphasized during a training program will improve.  For example, a program of long-distance endurance running will improve an exerciser's aerobic capacity, but it will do nothing to enhance the exerciser's performance on a heavy bench press."  ACE's Essentials of Exercise Science for Fitness Professionals

Balance in resistance, cardio, and flexibility training is vital to being physically fit and healthy!  Focusing on only one area is not going to fully benefit you and in fact will have a point of diminishing return regarding benefit.  Incorporating resistance and strength training along with cardio and flexibility training is the only way to assure you are physically fit.