First, Purple Rain is not really a musical. People sing to each other or the audience in musicals. There aren't even that many rock musicals, but Dreamgirls and certainly Hairspray are far and away better films.

I've heard great things about Tommy, but I've never seen it, Shit, even Head, as bizarre and tripped out as it is, succeeds where Purple Rain doesn't as far as having a core theme and a core plot, and even a consistant tone of lunacy.