High intensity cardio is good for your heart and lungs (among other things). Moderate intensity is good for burning fat. When you bulk, you eat more than you burn since your purpose is anabolism. Some moderate intensity cardio can help keep fat in check while you are bulking. But your body can't burn fat at high intensity, so it burns up carbs and muscle. Ronnie Colman is one of many body builders who have found that fat burning (moderate intensity) cardio allows them to eat enough to build muscle without putting on too much fat. So, no, it does not defeat the purpose. There are many health benifits to aerobic exercise that I won't get into, but you should not cut it out completely unless you are really lean and have a hard time gaining weight. If fat is an issue at all, you should work in some cardio even while bulking.