Yes, I see the humor in this node, and yes I understand that it isn't intended to be taken as a serious commentary on rap music or the industry, however, I thought I'd lend some perspective.
Although it's quite easy to view the entirety of popular music and culture through the filter of MTV (and VH1 or E! when Celebrity Deathmatch re-runs are on again), you're really only seeing a tiny fraction (and the worst fraction, at that) of the entire spectrum of life out there. If 50 Cent represented the forefront of hip-hop music today, I would readily concede that "Most rappers don't understand anything about music"; if Eminem was the most innovative rapper out there, I'd cave in to the mantra: "If you're white, stick to your designated musical genres." However, that simply isn't the case.
Underneath the apparently creatively inept rap mainstream is a thriving underground scene that produces art on par with its non-rap counterparts. The issue here isn't the musical style, it's the portion of it that is getting exposure. Pop, rock, country, jazz, and every other type of music have examples of mysoginistic, trite, sappy, violent, insincere lyrics. It's an institutional problem that stems from the fact that the average listener merely cares for a catchy melody or sample, not a lyrical depth bordering on literature. Eminem, 50 Cent, and DMX aren't products of rap music, they're products of the music industry. They know what sells, and that's what they deliver.
Just as Blink 182, Sum 41, and Good Charlotte misrepresent punk music, so do the above mentioned rap "artists" merely share a passing sonic resemblance with good hip-hop. The best of the genre is deeply entrenched in the underground scene, performing at small clubs, putting out cheap CDs, and setting the examples that the mainstream can then defile in the name of airplay and record sales. Artists like Sage Francis, Atmosphere, Del the Funky Homosapien, Aceyalone, Eyedea, Sole, The Roots, and countless others are making records far exceeding the quality of anything you'll see on MTV, rap or otherwise.
No way of playing music has an intrinsic moral bent or artistic value. Every new genre has had its puritanical naysayers and its bandwagon hoppers, but neither would be possible if there wasn't an underlying quality to it. Hip-hop is just a newer way of getting across the same feelings for which mere words will never suffice. We can stereotype and dogmatize until we convince ourselves that what we enjoy is the only "real" music, but that will only serve to impair us until even our internal arguments are falling on deaf ears. I dare anyone who believes the "bitches, guns, and bling bling" myth to actually listen to one of the artists I mentioned. I promise, there is good rap out there. Even if you're not an immediate convert, it might shed some light on the issue.
For more on the subject see: Hip-hop quadrant system