I agree completely with Hans, especially regarding the hodge-podge of dance genres in ballet schools. The idea behind the introduction of other dance styles, is that a ballet dancer needs to be well rounded to have a professional dance career.
Of course some dance forms do aid one in dancing certain ballets. It's good to know how to waltz if you are dancing Balanchine's Vienna Waltzes. If you are to dance a feasible Street Dancer in Don Quixote, it might be beneficial to understand the flair of flamenco.
However, emphasis isn't placed on modern dance training in ballet schools so that you can execute a proper contraction in one of Balanchine's works. Modern dance is studied because modern dance is an important part of a ballet company's repertory. The only reason that modern dance is in a ballet company's repertory is because no one has bothered to choreograph anything that is neo-classical. I find it hard to believe that we've gone as far as we can with ballet's evolution. All it needs is a little creativity. If I wasn't training to become a ballet dancer, I might try to choreograph something interesting that IS ballet related.
Rachel