Is it a possibility that you would consider a rapper who does not do any instrumental composition to be a brilliant musician?
While there is definitely an art to rapping over a beat, I feel that this art isn't nearly as complex as what other songwriters are putting together. I understand rappers are creating unconventional melodies in their songwriting, but I don't think that these melodies are nearly as consequential to the quality of a song. For example, while it is true that rappers must design and execute certain pitches in their songs, it doesn't seem consequential enough where I would expect that a rap producer often has to tell a rapper that his pitch was off during the recording of a song. And I can't imagine a rapper being told that his pitch was off after a live performance.
While people seem to believe they are trumping arguments such as mine by pointing out that rap lines actually do have melodies (as well as other complexities), they are failing to observe the inherent simplicity of these unconventional melodies, as well as the fact that these melodies are less consequential to the quality of the song. This is because the beat virtually always the main driving force in a rap song, and can cover-up lame rapping to at least some extent.
I also have similar views about vocalists in hardcore bands that only scream.