A student asked me "why learn about music?" and "why bother learn to improvise?" in this day of a thousand and one choreography DVDs.
The very essence of this dance is its connection with the music. Historically a dancer has danced to live music, much of which traditionally has been either wholly improvisational or has an improvised component. If you don't know what your musicians will play how can you choreograph it? If a dancer cannot hear the music and understand it, how can she interpret it? Improvisation is a foundation for much more than just on the spot performance. If a dancer cannot improvise, she can't begin to create music driven choreography for herself, and if she can't do that, then she'll forever be a lesser dancer than one who can.
A colleague once said "To abandon that skill in this dance may as well be it's death knell."