The way Yoga was originally taught in the East is one-to-one ...
... the guru would dedicate his life to his practice and meditation, and the disciple would live, work and learn directly from him, until the guru felt the student was ready to go off and explore things for himself, so he can in turn impart his knowledge to others. Shri Krishnamacharya (considered as the father of todays Yoga) was taught this way, and he taught Patabhi Jois (astanga), BKS Iyengar (Iyengar yoga) and his son TKV Desikachar (Vini yoga). It is the yoga taught by these three that has spread into the West and given rise to the phenomenon of yoga classes for the masses.
As effective as the guru/disciple system is, in our culture, most people don't have the time and money to work privately with one of the few teachers that have the experience and understanding to transmit the work directly. Also, I would say that the devotional approach to yoga - surrender to the guru - is not really appropriate for the average westerner ... Apart from anything else, most people haven't got the time or inclination to dedicate themselves to being with their teacher 24/7. Also, our culture believes in individualism and personal responsibility for our own development. So in my opinion, for the true transformational potential of yoga to reach the people that need it most, they are best served if they are given the principles to work with that will point them in the right direction, whatever kind of yoga they enjoy.
By giving you common sense principles to work with, a teacher can offer you the tools that empower you to take charge of your own wellbeing, bringing qualtiy and refinement to all that you do, whether it is yoga practice, walking, running, weight training or even standing in a queue at the supermarket. This I believe is really the best way to bring authentic yoga practice into alignment with the Western culture that is trying to assimilate it.
"Yoga… it is a living process that changes moment by moment, watching when we eat. How we eat, when we walk, how we walk, what we say and how we say it. All these things must be present in us and we must be passionately interested in them all."
(aka Marc Woolford)