I take it at face value. To me it is part of the genre. These people need to communicate themselves through song, and that's good enough for me.
I do ask for smooth transitions between song and dialogue which RENT so miserably failed at.
I do love stylized environments like SWEENEY TODD's as well. It makes it easy to "accept" the singing because they are part of a different environment.
Sometimes the material DOES need a "justification." I love the way Fosse used the songs in his version of CABARET which later informed Rob Marshall's vision of CHICAGO.
However, people forget about MOULIN ROUGE, the movie that I believe started to get people back in the theatres to see musicals. In that movie there was absolutely no justification, people just started singing because that's what they did, period. That's why I don't get the idea that you need a "device" to justify singing in a musical, same way as you don't need it on stage.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"