To me, a good score is one that tells the story through song and dance by setting the proper mood & tone. The score is the foundation for the lyrics, movement, and pace of the piece. A score can sound "good" on its own, but it's only "good" if it services the show. It's an equal collaboration with the book. When do we sing? When do we speak? When do we dance? In other words: both the composer and the playwright must collaborate to decide which moments of the story are better served with music.
Sometimes, the writer will decide that music is necessary for the entire piece. Other times, you get the opposite and have a play with songs. Or you have a play with underscoring. Or a play with absolutely no music whatsoever. It really doesn't matter how the story is told (music or no music), as long as it is told effectively. Just look at the story of RAGTIME. How did Doctrow's story move you most? Through page (novel), screen (non-musical), or stage (musical)?
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle
