I have never practiced yoga, but it is a type of meditation, and like all meditation I know of, the beginning point is to build tension and energy through pain to eventually burst through the black matter of the body and purge it clean. This purpose is not just for the body, but for the mind as well, and it takes great effort, motivation, and inspiration to do it correctly.
I'd first like to point out that yoga will not work for someone who does not have the mind intent to better themselves. A man can pose his body in any way possible, but this is not going to clear his spirit if he does not have the right intent. Second, most yoga instructors, do not even understand their art! For the average person, it takes years of training, self-sacrifice, and a seeking of body/mind knowledge to come to a glimpse of understanding what it means to be yogi. If most yoga instructors do not understand yoga let alone themselves, how are they supposed to teach another person to have an inner calm and to not have anger? On a spiritual plane and physical plane, they do not understand where this anger stems from, so they cannot destoy its source.
Science fails to measure the workings of the world because it limits itself to recognize only that which is "provable" by modern day devices, statistics, and to our limited senses. Using science to try and measure the benefits of yoga is just a joke. This is like trying to explain God through human language. The system is insufficient to even grasp an essence of the meaning.
It's not bad if you put it to constructive uses. Captain Kirk said he needed his bad side to help him get through the day ("Wolf in the Fold, Star Trek TOS.)
Properly used, anger can help you focus.
Although I don't think he used anger, the guy who built the Coral Castle probably knew a lot about focus:
I worship Arianna and you should bring her to Columbus OH for a killer power yoga class. BTW, I apologize for the rest of the state going red. Those of us with brains and consciences tried, we really tried. Ha-ha.