1. One of the principal benefits of residing illegally in the United States is, that one exists underneath the radar of the federal government, beyond the reach of tax withholdings, immune from many forms of prosecution, and in general invisible to the modern bureaucratic state, including that part of it tasked with deporting you. Thus, many illegals might choose of their own accord to remain illegal, especially those engaged in nefarious activities. At any rate, the illegal illegals would still be able to underbid the labor of the legal illegals, which would not disincentivize anyone from hiring the former over the latter. Obamnesty would come to rival Obamacare for lack of enthusiasm and participation.
2. One of the principle arguments urged in favor of some type of amnesty rather than enforcing the existing immigration laws, has always been the logistical nightmare of tracing and deporting several million undocumented people scattered about the country. By carpet bombing the illegal immigrant population with green cards, thus documenting the undocumented, Obama will have just made the first step in that process much easier for a future administration that is not so keen on having the country invaded.The likely outcome of all this will be a fizzle ultimately followed by a backlash when Middle America is forced by necessity to recover its confidence and its footing.