Despite what I may have written about high-risk exercises and “worthless” machines, I do not really believe there are any “bad” exercises. There are certainly exercises that are inappropriate for many, and just as many exercises performed in an inappropriate manner, but no exercise is bad by itself.
The key to avoiding an exercise that may not be right for you is to be clear about why you are working out. Training to improve athletic performance is very different from training to lower your blood pressure. If you are trying to lose weight, you will need to choose a different workout from someone trying to reduce back pain.
Know why you are performing each exercise and determine whether it will help you reach your goal (without injury). An athlete will need to perform fast and explosive movements; those trying to lose weight should be adding intervals (see posts from Feb. 2011). Do not assume that an exercise is good because the buff guy at the gym does it. Analyze each exercise and research its risks and benefits to find the workout that is right for you, and your goals.