Actually it's probably not just for PR, although that is a benefit. I think this is for his experience dealing with emerging urban markets, and the advice he can give. He has the connections to get square in the door in new places. That's the reason you add board members, not just for PR.

So, should we assume anytime a minority or women is asked to be on a board now, that it's for PR? Regardless of the success of the person? Tech is nothing new to Magic Johnson, he has been a venture capitalist in the industry for some time now. He's also part owner of the LA Dodgers.

Yes, hiring a successful entrepreneur and legitimate business mogul who is also African American is just a PR stunt.

Others have covered your assumption well, but how about this twist:

Successful and famous white male execs named to corporate boards are largely PR stunts as well. Board seats are often given out to lend credibility, not necessarily for business acumen.