Yeah, if all metal is just a form of rock music then I don't know what the point of music genres are anyway. Whether it's The Rolling Stones, Elvis, The Pixies, Def Leppard, The Rancontours, whatever you consider to be the definitive example of rock music, to say that black metal and slam death are part of that genre, I think that's being too marginal. Extreme metal typically has different song structures, vocal style, production style, image & lyrical content, and instrumental tones than rock. There's as much difference between metal and rock as there is between rock and jazz, blues and folk, classical and electronica, or easy listening and country. There's an overlap, yes, but there are many overlaps such as between rock & blues, country & folk, folk & blues, and rock & jazz. There's more in common between Neutral Milk Hotel, Lynyrd Skynyrd, Nine Inch Nails and Elton John than there is between any of those artists and Nokturnal Mortum.