That's a very subjective statement presented in absolute terms. Personally, I don't think of it in terms of "better" or "worse." It's two different art forms. Music is a language in and of itself. A piece of traditionally orchestrated music can be complete with no lyrics at all. Likewise, complete songs exist with only vocals and no accompaniment (vocal choirs and a cappella). In a traditional song, the melody is what takes us on the journey. Any lyrics or vocal lines must fit with the melody. Traditional music can also exist as a complete work without percussion. Hip hop (rap especially) is unique in that it is the words that are the journey. The beat and mix exist to embellish what is being said. The mix and the rap can't exist as a complete work without the other. Also, in hip hop, the beat is a necessity, melody is not. So to say that rock or metal artists can never compete lyrically with a rapper is like comparing apples and oranges. If I wanted to win a fight I'd simply reply that no rapper can measure up musically to a metal musician. That would be an equally subjective statement that misses the point that you can't judge a sculptor by how well he paints!