As we learned from Mad Men, the advertising industry is ever intent on harnessing trends that are bubbling up organically. Countercultural ideals (say, hippie-era peace and love) are wedded to a mainstream product (say, Coke) and blasted to the mainstream before anyone knows what’s hit them. Which is why hip-hop and advertising go together so well. Firmly rooted in youth culture, hip-hop is edgy but accessible, a combination that companies adore. It can be exciting to see your favorite rapper validated in big-name plugs, whether it’s Jay Z for Hewlett Packard or Eminem for Chrysler. But for every carefully considered,...