As I understand it, the biggest drawback to using any i series chip in the 13 inch MBP is that there isn't room on the motherboard for both the CPU and a discrete graphics chip. Apple used the C2D CPU in the current model because by doing so they could also use NVIDIA's 320M integrated GPU. If Apple shifted to i series chips Intel's restrictive licensing requirements would have required Apple to use Intel's vastly inferior integrated GPU. There may be other limitations as well but the conventional wisdom seems to be that the main one is the GPU difficulties using the i series would have created.