Not just cap but ignorance IMO.
There many, many people within Hip-Hop who are very talented musically.
Dj Quik, Dr. Dre, Battlecat, J. Dilla etc.
All of these guys play instruments (Piano for Dre),(Piano/Synthesizer for Quik),(Keys/Synth for Battlecat), (Drums, Keys, Synth for Dilla),
Ali Shaheed plays every fukking thing.
Also rappers should sing and singers should rap but you should respect both schools equally.
They're very different.
I sing fukking GOOD, like not "kinda" good but fukking GOOD but I still really took time
to learn and study the greats in Hip-Hop.