You need to distinguish.
There is music with a lot of rhythm, beat, that sounds "rap-ish" but is no more harmful than any pop music. In other words, the worst danger for such music is dissipation or working up the emotions.
But while it's true that dissipation is a bad thing, to avoid it there are MANY, many things that need to be avoided. Anything that breaks one's recollection after a morning spent at Mass, anything that pulls you down to earth and into worldly affairs is said to be "dissipating". Dissipation is similar to "distraction" and is the opposite of "recollection".
But then there's so-called "Gangsta Rap" which is what most members in this thread are referring to. Gangsta Rap is the kind not played on the radio, with that black & white "Parental Advisory: Explicit Lyrics" logo on the front cover when you buy it in stores. This genre of "music" involves copious use of swear words and/or blasphemies, boasts, promotion of immoral behavior -- mortal sins against 6th & 9th, as well as drug use, murder, and other law-breaking -- and an extremely degraded outlook on life, women, etc. Women are portrayed as objects for self-gratification. Pride and vainglory are the rule of life.
Yes, I know a bit too much about this genre.
(On an unrelated note, did you know that when you smash a CD with a rock, it breaks into many pieces?)
Anyhow, MC Hammer is an example of regular "stupid but innocent" rap, most of which I would call Hip Hop. Hip Hop is mostly a mid to late 80's phenomenon. All the hip hop I've ever heard is usually pretty dumb, cheesy, and not very attractive at all. Nevertheless, it's not any more "evil" than dumb pop music.
But a rapper like "Skee-lo" wouldn't be considered Gangsta Rap. It all has to do with the content.
Or a song like this, with a lot of rap-ish elements to it (it's in Japanese, but you'll have to trust me that there's nothing bad in the lyrics). Japanese works very well for rap, for various reasons.