I think the effectiveness of a musical film in comparison to a film score in a non-musical largely depends on the target audience of the film, genre of film, and plot of the film. In musical films like Grease, Chicago, Les Miserables, West Side Story, and Mama Mia, the musical and choreographed aspects of film work with the plot in an effective manner. I feel like [romantic] comedies (eg. Mama Mia and Grease) and dramas (eg. Les Miserables and Chicago) are the two genres that are best made in to musical films. Musicals are able to break down language barriers, telling the plot of the film through song which evokes plenty of emotion that is universally understood. West Side Story is a perfect example of a musical where choreography and music helps the audience to understand the story even though they might not understand every word of the lyrics. Musicals are able to effectively tell a story if the plot is more simplistic.
Contrary to [romantic] comedies and dramas, turning an action or science fiction film into a musical would be simply ridiculous. If we were to imagine an action film like The Dark Knight or Star Wars as a musical, I don't think these films would have been nearly as successful as they have been today. One reason for this is the target audience for films like Star Wars and The Dark Knight have the potential to be vastly different from the type of people who enjoy watching musicals like Dream Girls and Rent. The plot for The Dark Knight and Star Wars largely benefit from the music being comprised of a film score that is not classified as a musical. It would also be difficult to convert these genres into a musical because the plots are often too complex to understand if everything is sung. The scores in non-musicals serves to compliment the plot, not to tell the plot which can often be taken more seriously by the viewers.
“Summer Nights” (0:14:04) is the first song in the film, Grease, that gives us an