It is naive to ask why another? I was under the impression that Curiosity was supposed to be like the rover to end all Martian rovers. What else do they need? It seems like there must be something else to do aside from.... rove. Maybe not manned exploration, but something to push things further towards that?
Or are they still stuck on "We must make a final, irrefutable conclusion on whether life has or has not existed on Mars prior to introducing organisms to the planet"? Am I the only one who thinks that the search is kind of, well, silly at this point? How much of the surface have they covered with rovers? I'd imagine it would take much longer, and a much more exhaustive search to come to any sort of definitive conclusion.