I always like to stay for the entire credits (for movies I like) for several reasons.
1. sometimes there's something "extra" in the middle or at the very end of the credits
2. It helps me appreciate all the effort that went into making the film. All of these names I'm looking at had something to do with the creating of the film. So, even if they don't know I'm reading there names, and even if I don't know who these people are, I consider it an homage, or a courtesy. (Though I don't expect everybody else to stay)
and
3. The music at the end usually repeats themes from the movie, and for me it acts as a sort of recap of the film. I always feel like I'm interrupting the film if I leave while credits are still rolling.