I do not agree! This course shows us the different vowels to appreciate the differences of tones at first. I think it is the best method. I have advanced into lessons without using the listening exercises to finish the course very fast, but now I can hardly understand or make a good pronunciation in many words. So I think it is better to have a good approach with the pronunciation from the beginning. ;)
I think duolingo's model should spend more effort on the pronunciation at the start so it is learned the right way first. This way errors are not perpetuated/practiced/enforced during the lifetime of the course. The courses spend more time on phrases over pronunciation. How can it help with different vowels if you are not listening to them! You are missing out on the only resource they have for learning the phonemes, which is why finishing the exercises fast and without listening leaves you without a proper pronunciation.