Is Sweeney Todd even a musical?

Tim Burton seems to be casting his movie with an odd assortment of people. Okay, they're not odd for him, but they're odd for a musical. Now Johnny Depp did sing before I think in Crybaby. But even that seemed strange. Now he's going to use his girlfriend and friggin' Borat? I dunno. I love musicals and look forward to any big screen adaptation but why can't these people cast known singers who can definitely handle the task.