I think it also depends on what your goals are. If you want to compete, I think careful study of an individual dance is more important. If your goals are purely social, there is nothing wrong with working on a couple of dances at the same time. My goals are purely social, and where I take lessons, each 12 week course is 3 or 4, but most are 3. We often spend time on all three or just two of those dances in a 70 minute lesson. We have taken lessons for about 2 years now, and I feel like, as long as you practice and keep up, you can maintain those steps. If you want to get awesome at a dance, then I think careful studying of a single dance may pay off. I just think you need to evaluate your goals. If you are happy with where you are going, then that is all that matters.