What makes you think he can "hold down" second base? He was absolutely awful there in 2007/2008. Going by +/-, he was actually worse at 2B than SS. By eye, when he was with the cardinals he was a lot worse than his +/- rating suggests.

using UZR/150, averaging the last 2 seasons, he's like -5 runs at 2B and close to -10 runs at SS (you can't use last season's data for SS because he only played 79 innings there). those are pretty close to his career numbers: -4.4 and -9.2 respectively. essentially his value is the same at either position, because the positional adjustment difference between SS and 2B is 5, and that's close to the difference in his UZR/150. so Arizona should base this decision solely on the other players they can use at these positions.