Fiddler, to say that they're not heavy enough is like saying, "Nah, we shouldn't book Rush, they're not heavy enough." If they craft their setlist specifically for this event, it will be plenty heavy. Secondly, in a live environment, the energy/volume/heaviness is always upped. Thirdly, when you're talking about a classic band with great song-writing, great vocals, great lyrics, great musicianship, a huge catalog, a litany of great songs to choose from, and a progressive style that ranges from rock to metal, you're talking about a band that -- imo -- simply NEEDS to be a PP act.
"Man, if we book Rush, they might play 'Close to the Heart' and 'Cinderalla Man' and used all those synthesized keys that just aren't metal enough." Yet, we'd be passing up one of hell of a great band with plenty of prog metal tendencies.