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High temperature helps reduce the risk of injury while stretching, also, theoretically lets you stretch deeper. And while I agree with you, Bikram helps balance major muscle groups which is just as important as strength in general (i.e. strong quads + weak hamstrings = bye bye ACL). There are also lots of single leg balance/isometric holds that directly benefit skiing.
doing normal yoga you will workup a pretty damn good sweat, so the hot aspect does nothing more than dehydrate you(purifing you...lol) and test your mental fortitude. Its not better, not worse. just different strokes for different folks.
If you are trying to build leg strength, and you have an imbalance, yoga will do jack shit to correct muscle imbalances in your posterior chain when heavy weights are used for your muscle building(as they should be). Doing free weight work on your posterior chain is vastly, vastly superior. If you are relying on yoga poses to build your posterior chain, and expecting to it to balance out the muscle built from heavy squats etc, you are doing it wrong my friend.
Yoga is awsome for increasing flexibility, increasing your isometric endurance, and increasing your balance. It is a great tool to have in your toolbox, but it shouldnt be the meat and potatoes of your workout regimen. I personally use it about once a week, usually as a recovery day, or a day im not mentally up to lifting.
And the "old lady" yoga where all you do is focus on breathing and super easy poses is fucking useless. take a nap instead.