I'm not a fan of rap, and let me tell you why.
I listen to music for music. When I listen to music, I don't specifically listen to the lyrics. Sure, your brain still hears it, and subconsciously it has an effect on how you perceive the music, but what really speaks to me the most is the music itself. Maybe I'm just weird or something, but sometime music makes me short of breath, or gives me goosebumps, or even makes my eyes water. And it's never because of the lyrics, or because I agree or sympathize with what the singer is singing about. No, the only time I learn lyrics is when I want to be able to sing along and not look like a retard. No, it's not the lyrics, is the emotion that's built in to the music itself.
If you listen correctly, you can feel exactly what the composer was feeling when he wrote the song without worrying about the damn lyrics. This will sound incredibly conceited, but I've written music like this before, too, and people have told me they've gotten similar effects sometimes (chills, etc). Hell, sometimes I listen to my own music and get those feelings. It comparable to being on some kind of drug, maybe, sometimes. But my point here is that I pretty much never write lyrics, and I still get the point across.
In this regard, what value does rap music have? More than 80% of the time it's just the same beat looped over and over again while some guy talks about something and says "yeah" or "what" 73 times. Not that rap artists don't have any talent or anything, but it just doesn't speak at all to me, personally.
In my opinion, there's only two types of music.
The kind that lets the music speak, and the kind that lets the lyrics speak.
Sometimes the music carries more emotional/whatever value than the lyrics do.
Sometimes the music is stock, boring, ordinary, and the lyrics are what make people like it because they can relate to it or they think it's catchy or whatever.
If they have both, then great.
But as for me, personally, I'd much rather the music speak to me than the lyrics.