Would you like a Big Mac and Fries with your supersized Yoga?
In America especially yoga is misunderstood because it has been been cocacolonized to the point it is just another way to appeal to body consciousness. In fact what is practised in most yoga "Studios" is just hatha-yoga a subbranch of rajayoga, which was long done for good health and better digestion, etc. This training should be handed down with an appropriate teacher/guru who is qualified to teach under the systen of Patanjali but unfortunately it doesn't happen all that often for many. What is left is that so many people are doing it to loose weight and fit the size 4 dress. Blame the teachers, not the audience. They are no better then a preacher driving to his church in the ghetto, wearing a five hundred dollar suit, and riding in a white Cadillac. It's the same thing.
In the Vedantic system there are four main yogas, or method of yoking to Brahman. There is Bhaktiyoga, which is yoga of devotional love. Karmayoga involves a daily practise of selfless actions. Jnanayoga is the yoga of discriminiation: trying to discern truth through the illusory world we exist and interact in. Rajayoga is meditative. All four paths are heading in the same direction.
What isn't understood is that many yogis in the east will have nothing to do with hatha-yoga because of the fact it breeds attachment, which is the opposite goal of the vedantic pursuit, the idea of fully understanding the "Self" or "Atman". Fortunately there are a few organizations in America (for instance) that are still able to teach the "other yogas" such as Self Realization Foundation, Vedanta Centers, and Sarada-Ramakrishna-Vivekananda Association among others (there are many others but I am not familiar with most of them.. there are a lot of easy to spot con artists out there).
As for worshipping other gods, the idea in yoga and vedanta is a nondual one at the end, in that all is God, and god is all. There may be items of devotion, such as Ma Kali, Jesus, Mother Nature, Krishna, or Jehovah, but in the end even these fade away into articles before the supersoul ideal of Advaita (nonduality). Easy to say but almost impossible to fully realize, so that is why there are so many methods of trying. There is a lot of statements made by fundamentalist Christians stateing that yoga is "Hindu" and "pagan". This does not hold water, really, unless anything they don't like can be labelled "pagan". In fact most religion in the subcontinent is about as dualistic as Judeo-Christian-Muslim religions. Advaita-Vedanta (nondualism), while upheld as the highest ideal, is not really practised much, per-se. It had the benefit of being brought to America as early as 1893 by Swami Vivekananda, and then later by many others.
The witnessesa have always been worried of yoga because of this fact. Even hatha yoga can make one more interested in yogic practices. For instance many yoga practitioners begin their practise with a short mantra "SoHam" which is Sanskrit for "I am He" which is an advaitic statement. SoHam is easy to remember in breathing excersizes, also. There is no real transaltion for the term Brahman, but the gist of SoHam is "I am the Divine", and is very close to another prime Advaitic statemtn:
Tat Tvam Asi: Thout Art That.
It is easy to see why the covetous JW organization does not want its members trying yoga and getting these ideas.
So I could say I've been doing yoga a few years, even tried hatha once, and yes I have prayed to many deities. It's good for the mind, to my thinking. As a theist, I can't really see God as this everpresent nonpersona reality. I need something to think about when I am in the "religious" mood. On the other hand I can't think of a single reason why anyone would want to sleep on nails, unless they are some sort of sideshow performer. But make no mistake, Yoga is absolutely spiritual. If it isn't, then it isn't yoga at all.
Aum Shanti.