Yoga is not about poses. It is not about breathing.
Yoga is about consciousness.
I read this in the Yoga Journal about a month ago and it has really stuck with me.

The Raja path of yoga or "King's Yoga" outlines eight steps toward Self-realization, the third of which is asana.
Of course, most people who have delved into the philosophical aspects of yoga realize that the asana portion of the 8-Limbed Path or the "practice of poses" is a mere eighth of the work to be done on the way to enlightenment. And depending on who you talk to, the poses were designed mainly to prepare the body for long periods of meditation, or to give 13-year-old boys a way to release some energy while studying to become yogis.

Unfortunately, for many modern Westerners, it's easy to get caught up in the physical practice--a lot of people think that becoming flexible is the goal of yoga. Now to be clear, I don't think there is anything wrong with asana practice or with using it as the stepping-stone toward a more well-rounded practice. It is a fairly accessible format for the modern-day yogi and has allowed me to begin inching my way along the 8-limbed-path.

In fact, there are ways to use the asana practice in order to advance oneself toward expanded consciousness. The practice on the mat is the first place a lot of us begin to feel aware for the first time. It can be a huge wake-up call for a lot of people. Just gaining body awareness, something many modern people lack, can be a necessary step. After the body awareness comes the breath awareness, which can initially enable you to feel more present in the moment.This moment-to-moment presence, which can take years to cultivate, is the main goal of your work on the mat--not to put your leg behind your head or to hold handstand for ten minutes.

Another goal of the asana practice is the increased flow of prana or "life-force." Prana as a term can represent several things, but in an expanded understanding can mean vitality or aliveness. When one practices postures and links movement to breath, energy channels throughout the body begin to "wake up" and make space for unhindered flow. This is one of the reasons that you feel so good after you practice--you know that sense when you just get done with class that you really want to go back, but you're not quite sure why? That is mental awareness of increased prana, or more simply feeling more alive.
When you can begin to tap into your own individual flow of prana, you can begin to apply this new-found vitality into your present moment awareness.

While the body does become toned and supple as a result of years of practice, it should be considered a by-product (albeit a nice one) of the third step (asana) on the way to the eighth step (samadhi or self-realization).
I can greatly appreciate the modern-day application of asana because it brought me to the path of yoga. But after awhile, we have to make that next step. We have to accept that the ancient science of yoga aims at a much bigger goal than standing on our head. And that goal, of course, is Self-Realization.