On further consideration, I think yoga is best thought of as a sport, rather than a therapy. You risk injury in all sport activities, but you decide it's worth it, not just for the physical benefits but because you enjoy the challenge, it makes you feel good about yourself. Yoga definitely gives benefits that are more than physical, and like almost any physical activity, you have to understand the risks, and especially be very aware of your level of competence to avoid pushing your risk profile dangerously high. The teacher is really a trainer, and should be competent enough not to have you do something that will hurt you. This is hard in a big class; the best teachers constantly remind you to listen to your body, and be aware of your limits.

It should definitely not be used as a replacement for physical therapy, though it can help with general problems such as joint and back stiffness and mild pain.