"Criminals", in that they flout our immigration laws. Wanton disregard for a basic law such as immigration does not bode well for the illegal's observance of any other US laws. If the illegal will not obey the immigration laws, he will very likely ignore any other laws which he wishes to disregard.
We already know that not the best nor the brightest are the vast majority of illegals. Jimmy Carter allowed in the scum and dregs of Castro's Socialist "paradise" and prison resort during Mariel. We seem to forget the thousands of crimes committed and the enormous disruption of civil life because of that wholesale influx of otherwise illegal mass. Vincente Fox goes Castro one better. He gets a president to agree to take his overflow of human surplus, thereby relieving himself of a teeming mass of unemployed and itinerant drifters who might otherwise join Mexico's "People's and Peasants" guerrilla movements. Instead of forcing Mexico into the 19th century and making Mexico respond to its fundamental weaknesses, we take them and thereby allow a latent cancer to infect our own economic, political and social institutions.
As if we don't have enough troubles to correct.