I don't quite understand where you're coming from. JD is not Stephen, Stephen is not JD. He was coming off the best season of his career prior to the injury, and he had never been injured for a long stretch of time before it. He was also an above-average offensive and defensive SS (stats have no bias, and they contradict you in that sense). The comparison spits in the face of logic.
As for "why was he signed", he was the best option available on a one-year deal. No one thought at the time that Iglesias was ready to take on an everyday role, and anyone who says they did was either delusional or flat out lying.