This is a question we are asked all the time, which surprises me as I had never even considered yoga as exercise. But of course it is. It's in every gym, and loads of people practice it purely for the physical benefits. And that's okay with me :)
What many yoga students in the west don't understand is, yoga is actually a school of thought, with its main goal being self awareness. The practitioner can achieve this goal through many paths of yoga, including asana (the physical postures), meditation, chanting, prayer. Not all yogis downward dog.
So I suppose when it comes to weight-loss, it depends on what path of yoga you are practicing. I believe that If you are usually inactive, and you begin practicing asana (the physical postures), then simply because you are moving you may lose weight, but generally, yoga is not considered a 'fat burning' form of exercise.
For cardiovascular training or fat burning, you would need your heart rate to be up at a moderate to high intensity for the majority of the class, so you could potentially lose weight if you practice asana 4-5 times per week at a high intensity (Ashtanga, Power Vinyasa etc) together with mindful eating.
Also, when we practice asana, and other paths of yoga like meditation and breathing exercises, we are decreasing our stress levels, and when we commit to a yoga practice, we tend to make better choices in our diet and our lifestyle. So yoga can indirectly help.
So my answer to this question is, educate yourself as to what weight loss actually is physiologically, then decide whether you want yoga to be that path. Personally, I want yoga to be my path of self study. My practice of non-attachment to ego, and everything I identify with. I respect the ancient art of yoga and the fact that it's an internal practice. Yoga is my personal journey of self awareness. Yoga is surrendering. Yoga is my state of mind.
For cardiovascular exercise, I box and jog :)
The Yoga Social Team