Just to get this out of the way, I'll start by giving my answer to this question. My answer is: It depends. It depends on what you take yoga to be, and what you want yoga to do. The next part of this post is devoted to trying to make sense of this answer in a very long-winded kind of way. If you can't bear to read long-winded responses, and don't want to know my reasons for thinking why yoga may or may not be about self-expression, you are welcome to stop reading now :-)
Let me begin by saying that there do seem to be good reasons for thinking that yoga is about self-expression. After all, there are more yogas in the world today than there are fingers on my hand (duh!). These range from:
(1) more "traditional" styles that claim to be following lineages that can be traced all the way back to Patanjali and the Yoga Sutras (Ashtanga, Iyengar, Sivananda, Kripalu, the Himalayan Institute, and a whole bunch of styles that I cannot call to mind right now), to
(2) contemporary off-shoots of these traditional styles (Power Yoga, Vinyasa Yoga, Trance Dance, and a whole bunch of off-shoots that I am ignorant of), to
(3) a whole bunch of contemporary yogas that serve to fulfill certain particular functions or certain particular fetishes (Yoga for Golf, Yoga for Runners, Yoga for Chocolate Lovers, Yoga for Wine Lovers, Yoga for Chocolate and Wine Lovers, Yoga for Chocolate Lovers Who Hate Wine, Yoga for Wine Lovers Who Hate Chocolate, Yoga for Better Sex, Yoga for People who want to have less sex... The list goes on and on. It's simply impossible keep track of all these yogas these days. It seems that everywhere I turn, there is a new yoga or two springing up somewhere...).
I hope you will excuse my long-windedness here (I did say you could stop reading, right? ;-)). But there is a method to this seeming madness. Let me start by observing that the move from (1) to (2) to (3) is roughly chronological in order. Many of the yoga styles that are based in some lineage or other ((1)) were created by their creators either sometime in the nineteenth century or in the early part of the twentieth. The offshoots ((2)) were mostly created in the last twenty to thirty years (think Beryl Bender Birch's Power Yoga or all the many vinyasa yogas out there). Lastly, unless I'm mistaken, what I call the function- or fetish-oriented yogas ((3)) only appeared on the scene somewhere around the last ten to fifteen years (maybe even more recently).
Is there a reason for this chronological order? I don't know... But I'm quite sure somebody could look at (1), (2), and (3), and say, "So how is something like Yoga for Runners really all that different from, say, Ashtanga Yoga, when all is said and done? Sure, there are different poses in each yoga, and one yoga prescribes a certain order in which to do them while the other doesn't. But what's the big deal in the end? Just as Pattabhi Jois or Krishnamacharya took some moves from Scandinavian gymnastics and Indian wrestling and wove them into a particular sequence of postures that they claim is beneficial for people and called it Ashtanga Yoga, the creator of Yoga for Runners also took some yoga moves that she claims is beneficial for runners and called it Runner's Yoga. When it comes down to it, isn't it all about taking a bunch of otherwise disparate things, slapping a label ("Ashtanga Yoga" or "Runner's Yoga") on them, and then marketing the resulting product as something that will benefit you? And if each yoga turns out to benefit at least some people, what's the problem? And if marketing a product amounts to creating something, we can also say that Krishnamacharya and Pattabhi Jois and the creator of Runner's Yoga are all expressing themselves. Therefore, all yoga is self-expression. Q.E.D."
I'm sure somebody could respond to the above argument by insisting that what people like Krishnamacharya and B.K.S. Iyengar and SKPJ are doing is very different from what people like the creators of yoga for runners or yoga for chocolate lovers are doing. The former are taking a particular tradition (the yoga tradition, as outlined by Patanjali) and interpreting it in a particular way in order to create practices that allow people to connect to this ancient tradition, whereas the creators of yoga for runners and yoga for chocolate lovers are simply catering to functions and fetishes that are unique to our post-industrial world without attempting to come up with some kind of narrative about what enjoying chocolates or running well has to do with the yoga tradition.
But something tells me that such a response probably wouldn't cut much ice with the person who believes that all yoga is ultimately self-expression. After all, it could be argued, what kind of difference does what one takes as one's starting point really make? Why should it matter whether one's starting point is to create a practice that makes Patanjali's yoga sutras relevant to the people of today, or to create a particular yoga that caters to people in the throes of one particular passion or fetish? One way or the other, one is responding to an urge--a creative urge--to fashion something out of the things already at one's disposal. Why should one starting point be deemed to be "better" than another? Aren't all creative urges equal?
The above exchange could go on and on, without either side putting forth a really decisive argument. Why is that? Could it be that each side means something different by the word "yoga"? In her book, Yoga PhD, Carol Horton points out that what we moderns (or post-moderns) take yoga to be has evolved drastically from what the classical brahmanical conception of yoga was. Whereas the brahmins of classical Indian society understood the objective of yoga to be moksha, or liberation from the endless cycle of birth and death, the average person who takes yoga classes at a studio or gym today is not looking to free herself from the endless cycle of births and deaths (she may not even believe in reincarnation). Indeed, we can even go further and point out that even many of today's adherents of the yoga lineages in (1) do not have moksha as a goal; it is possible to practice Iyengar or Ashtanga without believing in reincarnation.
Horton notes that for many (maybe even most) of the practitioners of modern (or post-modern) yoga, the goals of the practice are more this-worldly in nature. They could range from something as immediately physical as relief from back pain, all the way to something less tangible like equanimity in the face of the challenges of contemporary life. Whatever the case may be, it is clear that the goal of the typical modern yoga practitioner is of a totally different metaphysical quality from the goal of the brahmin living in classical Indian society. And since goals define the nature of the activity, we would also have to conclude that the yoga that is practiced by us moderns (or post-moderns) is not the same yoga as the yoga that was practiced by the brahmin in classical Indian society. The same word ("yoga") refers to two different activities that may have some similarities when seen from the outside, but are ultimately very different in nature. While it is possible for some practitioners of modern yoga to have some form of self-expression as one of the goals of their practice, self-expression was quite definitely not on the radar screen of the brahmin in classical Indian society. For the latter, the practice of yoga was a roadmap and a vehicle for getting to moksha, no more, no less. So the brahmin in classical Indian society had a very different idea from us moderns of what yoga is supposed to be, and what it is supposed to do. Hence there can probably never be any agreement in the debate over whether yoga is about self-expression.