Uh..the reason rappers "cant sing", is because they don't sing..they rap
Rap can, and is quality music. You can't dismiss it because you don't enjoy it, or because you think it's too popular or whatever. A lot of eminem's lyrics are pretty fucked up, I agree..but they are pretty much all genius, and a lot of kids can relate to them, thus he sells records. Eminem represent's *mainstream* rap, and he does it extremely well. I don't know anything about underground rap, or freestyling, but everyone's got their form of expression and if you can't respect it you probably shouldn't talk about it.