Let's begin with what I call the official line: ""Asana is not important. The most important thing is to breathe and to be aware of everything we are doing, both on and off the mat. The asana is just a means towards this end. Fixating on the achievement of asana brings up pride, ego, and all the other things that get in the way of self-realization."
Stephanie responded to this official line by disagreeing that asana is not important. It may be true that, in the big picture, asana is a means towards an end, but it nevertheless serves an important role as a means. She puts it this way, "It *is* a means to an end. The bonus features of practice are what's so addictive and fun and yet challenging about it -- the energy flow, the dance of vinyasa, and the mental focus, the attitude and approach to challenges, the simplest things becoming complex, and the refreshment that comes with those deep inner releases…"
I like the term "bonus features"! Makes doing the practice feel like watching a DVD :-) But there may be more to these "bonus features" than one might suppose. Think about it this way. Most of the time, you can watch a DVD movie and skip the bonus features, and still get everything you can reasonably be expected to get out of the movie, if you are fully attentive while watching the movie (some movie buffs may disagree with this, but whatever...). But what would asana practice be without the "bonus features" that Stephanie talks about? I think many of us would agree that it would be nothing more (or maybe even less) than gymnastics.
So these bonus features are more central to the practice than one might suppose. Indeed, things like developing mental focus, finding deep release from certain emotional and physical blocks, and having a certain attitude towards challenges embody many of the attributes one seeks to develop from any kind of spiritual practice. This suggests that although asana is a means towards an end, it serves an important role in this capacity, because it is a tool that we can use to activate all these valuable "bonus" features (No offense, Stephanie, but I'm beginning more and more to feel that "bonus" features is really not the right term here, but I can't think of anything better right now, so I'll stay with this term).
This is not to say that asana is the only way to activate these bonus features; as we all know, many other spiritual practices have other ways and means to activate these bonus features without asana or any kind of physical practice. But within the systems of hatha yoga (and Ashtanga Vinyasa yoga in particular), asana practice is indispensable as the "activator" of these bonus features. Without the asana practice, we lose a valuable means of accessing and actualizing the repository of spiritual knowledge hidden in the depths of the yogic tradition.
We can also look at this issue from a different angle. The 12th century Chinese Confucian scholar Zhu Xi says, "When one knows something but has not yet acted on it, his knowledge is still shallow." Because of this, he continues,
“The efforts of both knowledge and action must be exerted to the utmost. As one knows more clearly, he acts more earnestly, and as he acts more earnestly, he knows more clearly… It is like a person’s two legs. If they take turn to walk, one will be able gradually to arrive at the destination. If one leg is weak and soft, then not even one forward step can be taken.”The Complete Works of Zhu Xi, translated by Wing-Tsit Chan
Zhu Xi was speaking of moral knowledge and action, but his remarks can also be applied to yoga practice. One cannot credibly claim to have any access to any kind of spiritual knowledge if one has no way of acting on this knowledge and making it manifest in this world. Knowledge is activated by and informs physical practice: As one grows in spiritual knowledge, he applies himself more earnestly to the asana practice, and as he applies himself more earnestly to the asana practice, he comes to know the nature of reality more clearly and deeply. If either one of these "legs" (knowledge or practice) is "weak and soft", progress is impossible. I think it is also with these thoughts in mind that B.K.S. Iyengar remarks:
"[w]hat I teach is spiritual practice in action... I use the body to discipline the mind and to reach the soul. Asanas, when done with the right intention, will help to transform an individual by taking the person away from an awareness of just the body toward the consciousness of the soul. Indeed, as I often say, body is the bow, asana is the arrow, and the soul is the target."