Why do actors and directors still value Constantin Stanislavski's system today?

I think because some of the greatest actors of our time are students of this system. Like, Jack Nicholson, Robert De Niro and Al Pacino. Every time I watch a DVD , I watch it twice . First I just watch the movie and then I watch it with the director/actor(s)/commentary on. I say that to say this, in the commentary for the movie "The Departed" they were talking about how great Jack Nicholson was in using Stanislavski system to improv many of his lines and in the directors opinion made the movie better.