I was trained in a very spiritual yoga school. I received fantastic training but after a while I began to feel coerced to adopt a belief system that I didn’t necessarily agree with. Now I teach freelance at a few places, and teach Silver Age yoga for seniors. It is important to me that each student be able to accept and enjoy yoga on their own terms. If they enjoy the physical well being, they will continue to come to class and eventually they may benefit from the calm that control of breathing can offer them, as well as coming to understand more about their own bodies. Over time they will learn more about themselves and their abilities. Whatever form that takes for each student it is always a positive experience. To say that yoga must be a certain way is to disrespect the very nature of yoga. If you embrace all the limbs of yoga you will incorporate a spiritual practice. However that is not a necessity and this kind of inflexible (pardon the pun) approach turns students (and even teachers like me) off where a more open minded approach can allow everyone to realize benefits in their own way.