I think there are two situations here. One is competing or performing. If it's a good dress, which doesn't bring attention to your technical flaws, fits you perfectly and is comfortable and doesn't get in the way, then not only it makes you look better, it also gives you the confidence boost. Another is lessons/practice. Personally I see nothing wrong with wearing yoga pants for lessons or practice. I would say that's what I wear for my lessons 80% of the time, and about the only thing I practice in (I've been practicing in shorts lately because it's been so hot). I know what a skirt feels like, and I don't think I dance differently on a lesson when I am not wearing a skirt vs. when I am (if I did, I certainly would hear about it).