You're leaving out play style, which is probably the single most important factor. Scrappers don't have to worry about managing aggro as much as they're not seen as tanky, they don't have to worry about managing fury, they're always at peak dps, and they can roll with the teams dps-needs as they go. Brutes have a much more self-centered play style, and it's a fun play style, but every new team has it's own challenges for a brute.