Rap songs tend to me more clear, not just vocals, but song as a whole, because we don't record the instruments, we use mastered samples,or mastered beats.
So we record the vocals, as clear as we can, and viola.
As for the bad mouthing of rap, I wonder just who you listened to.
Granted, some artists have messages of nothing more than getting rich, shooting at someone, and ho's, while others actually have a message behind the song, ie- 2Pac, he is one artist who had a message behind every single song he ever made, and those who have lived a poor life, or seen those struggles, can easily relate.
He didn't rap about killing someone, just to rap about it, he rapped about what HE went through, what HE seen, his emotions, the internal demons he dealt with.
Although 99% of you couldn't relate to this, as you've never been brought up to see such things.