Using aerobic activity to help you get cut up makes sense. If you me tabilize an extra hundred calories doing cardiovascular exercise, that is another hundred calories contributing toward reducing the body's fat stores, or another hundred calories of, say, valuable protein you can eat while continuing to lose weight on your contest preparation diet.
However, the body's ability to tolerate the stresses of aerobic exercise is not unlimited, too much cardiovascular exercise can end up being detrimental. Excessive aerobics (and there are those who have tried doing endless hours prior to a contest, to their later regret!) can cut into the recuperative ability of the muscles involved and the physical system as a whole, leading to the scavenging of muscle tissue for energy (using the larger white fiber as fuel for the smaller red fiber), and resulting in inducing a state of overtraining.
Overtraining doesn't mean simply being tired from too much training. It is a condition you get into from too much exercise over too much time in which certain mechanisms in the body that supply you with energy and allow your body to recuperate are depressed or shut down. Over training is a chronic state in which you just can't perform no matter how hard you try. If you find yourself overtrained, the only good remedy is rest, sometimes weeks of it. But you can avoid the overtraining syndrome properly scheduling your training, making sure you get enough rest and enough nutrients in your food.
But one good way of preventing overtraining is not to go overboard on the cardiovascular training. Remember, to look like a bodybuilder you, need to train like one. To benefit from the concept of specificity of adaptation, you need to make sure that the main influence shaping and developing your body is progressive resistance weight training pumping that iron, not aerobics.