Harem pants are so cool! I have just ordered two of them also from Amazon and can't wait to get my hands on them. Saying this, I doubt they would be stretchy or comfortable enough for all yoga asanas so I dont think I'll wear them in class. Not sure how far they will stretch for warrior 1 or swan for example. More importantly the teacher or student needs to see the exact positioning and alignment of feet and hands so my question is how easy is it to spot alignments with harem pants??
But if they are practical and they look cool on you then go for it. Who knows you may be setting a trend and everyone in your class may turn up in harem pants next day.