At least part of this blogsphere movement is based on another typical media meme, the grass is greener somewhere else. Trevor Plouffe is on a hot streak at AAA and suddenly has gone from"failed prospect" to major league savior in just two weeks, or two months depending on how vociferous the blogger was about his failure. Plouffe, of course, struggled with both the bat and in the field in a limited audition earlier this year. Nishioka's move to shortstop was at least in part driven by Plouffe's failure to claim the job. Plouffe however has been on a tear with his bat at AAA recently while playing some in right field in addition to shortstop. I have always been a fan of Plouffe's and his performance at AAA is not really a surprise. He is finally maturing and showing the power that was projected to be there. At some point he will be ready to contribute again at the major league level, but its not clear that will be at shortstop. It was his fielding, not his bat, that got him sent back to AAA. Its way too early to decide he is ready for another audition. In any case, I don't think its time to give up on Nishioka to make room for him in the big league lineup.
Nishioka needs some time to adjust to the big leagues. If he is still struggling in a month or two, then you might decide the experiment was a failure and send him to AAA. But sending him to AAA now is the kind of snap judgment that makes for good stories in the media, but lousy baseball teams.