I spent five years taking ballet class every day, and playing the harp as well. I would say it can strain the shoulders, as second position for the arms is about the same as harp position, and holding your arms there more or less for 90 minutes a day, in addition to practicing, is not the best. You at least need other exercise to build strength aside from muscle tone. It is good for rhythm certainly, and my ballet teacher taught me a lot about expressiveness. I recommend that you try to see the new movie about the Ballets Russes, as well as The Red Shoes.