Having said that, I don't think a good instructor should insist you try anything, and I didn't mean to imply otherwise. Karina didn't feel comfortable trying that pose, and that is a reasonable response for a student to have. I'm just saying that in a typical yoga class, an instructor is going to mix it up with some easy stuff and some challenging stuff.
"Marge, if you're going to get mad at me every time I do something stupid, then I guess I'm just going to have to stop doing stupid things!" - Homer Simpson in the Mr. Plow episode
I don't just do yoga. I also walk 2-3 miles a day, for cardio. All exercise programs should combine cardio and muscle building.
As for ballet, most dancers do Pilates, which is very similar to yoga in the way it slowly stretches and builds muscle.
Healthy food is a precondition to get proper results, at least for me.
Anyway, what you said above is way I see yoga as a supplement but definitely not the only form of excercise that should be done. (However, better yoga only than nothing at all )
I also do Pilates and just like Yoga I find it helpful for the ballet classes.
But none of these help me with my stamina, that's why I go running a few times a week or do body toning and similar classes.