Let's start with a youtube clip from Ally McBeal -- James Marsden (X-Men, Superman Returns) sings "The Lady is a Tramp."
I know why everyone hated this show but I think it had a lot going for it and wanted to offer a defense of it on a single point. David E. Kelley is the only guy working to keep the genre of the American Musical alive after it got eaten by Disney cartoons. Mulin Rouge was fun, but a dead end (it took that kind of pastiche about as far as it's going to go). Ally McBeal attempted to keep the whole thing fresh, and injected life into the genre by fusing it with the weekly legal drama, and by incorporating the singing into the narrative. His twist is that, rather than using song as a kind of metaphor for an inner state, Kelley's characters are aware that they are singing -- at their after-work bar, in a hallucination, in their fantasy life, in a church choir, as part of a courtroom demonstration. Kelley had earlier failed to keep the musical alive in his short-lived bomb Cop Rock, but he did a much better job here creating the kind of whimsical background necessary to justify constant singing. His characters are romantics who prefer illusion to cold reality. His favorite senario -- here, on Picket Fences, on Boston Legal, on all his shows -- is the quirky, half charming mentally ill person who may get more out of life than so called normal people. Taking song seriously becomes, in the show, the imagination defending itself against a universe of death, as in the poems of Wallace Stevens.