I think working out is healthy. If it becomes an addiction I don't see how it can hurt. Unless a person doesn't stretch or keep the right diet plan you can't go wrong with this. Working out will boost your immune system. The only way this kind of addiction could hurt you is if you do the exercises wrong.
I can imagine those who would want to achieve a buff body exercising their ass off the gym to the point of excess. I think that when it comes to exercising, it's pretty easy to know when you are doing it excessively already when you feel tired and yet you still want to exercise.