Hmm. Well. First of all, moderate cardio has no impact on overtraining. One must be performing chronic amounts of cardio to achieve an overtraining state. Generally, aside from being tired, pure cardio won't affect your muscular state noticeably, the exception being if you pick a stairmaster or something that taxes a particular muscle group.
Secondly, cardio doesn't burn up muscle protein any more than pure dieting does. Any form of caloric deficit, whether diet or exercise induced, can consume muscle for energy. This can be minimized by consuming whey before any form of exercise, including cardio.
Also there's some evidence that cardio burns subcutaneous (below the skin) fat better than pure dieting, which tends to burn visceral (around the stomach organs) fat. So it does make sense to do cardio, especially when one is already quite lean.