Maybe Kurt Cobain and Johnny Cash's respective wills demanded that their spirits live on in the form of a digital Village People cover band, meaning the option to export Band Hero songs into Guitar Hero 5 isn't totally bad.
On the other hand, seeing Cobain, Cash and Muse frontman Matt Bellamy performing "YMCA" or, say, The Spice Girls' "Wannabe" is the kind of thing that some people may find objectionable. Who knows?!
Even if Nirvana and Johnny Cash fans—or those in control of their estates—do find this sort of virtual karaoke circus act in bad taste, just remember that to prevent this kind of stuff would be nigh impossible. Limiting the ability to use certain Guitar Hero 5 characters in other performer's songs would require lines of programming that could number in the teens. The teens, people!
Who wants to place bets on the copyright claim takedown notice?