It's interesting to think about whether "rushing" a prospect to the bigs damages him somehow if you do it too soon. Three types of examples in recent Cubs history illustrate the different ways this can work out:
1. Starlin Castro - brought up quickly, probably largely to create fan interest, and has struggled in parts of his game, but generally been a useful player, who's good skills generally overshadow areas needing more development
2. Brett Jackson - brought up roughly on schedule, and tanked completely in the majors. Was probably brought up without a full 2012 in AAA on the strength of his second half 2011 performance, which featured a troubling 30% K rate, but which looked very good on the strength of an unsustainable .402 babip. A dramatic lower babip (.298) in Chicago, combined with a predictably elevated 42% K rate (it was 34% at Iowa) sealed his fate. And now, he appears to be just lost... irrevocably broken perhaps? If he had just reverted to his 2012 Iowa numbers, you could say that's just his peak, but he's way worse, suggesting that the experience with Chicago could have damaged him in a non-baseball-ability way.
3. Anthony Rizzo - who also tanked miserably when first brought up, but seems to have mentally survived the experience, and is afforded the chance to learn how to hit in the majors via on-the-job training because his club is going nowhere, and can afford to have him struggle there. His hitting performance - in many phases - has been encouraging, despite some pretty quiet stretches. Rizzo might not have been out there every day on a contender...
So, if the Cubs are no good in 2014, I guess you could make the case that Baez might as well learn baseball playing with the best players, if you can afford to wait out his growing pains. His K rate, so far, is a bit higher at Tennessee than Jackson's was, but he's also younger than Jackson was, and has more power, so there's no reason not to put him on the half-season at Tennessee, half season at Iowa, September callup for 2014, assuming things progress. But there's also a case to be made, if the Cubs aren't competitive, to try skipping that AAA move and trotting him out in Chicago every day after the All Star break. Assuming that he will struggle mightily, like Jackson, Vitters and Rizzo, will he emerge in 2015 better for the experience and coaching? As long as you don't psychologically break the guy (if that's what's even wrong with Jackson), why not?