Hiphop is different to singing I think.Singing should sound good, hip hop doesnt have to. Hip hop has to have a message, singing doesn't have to. Basically, the way I see it is that singers can have 'writers', but should never have 'ghost writers'. Hip-hop artists shouldn't even have 'writers', let alone 'ghost writers'.
Rappers can have 'writers' if the track is only meant to sound nice (like I see P. Diddy's work as), but still - never 'ghostwriters'.