It would be nice if we could live in a world with open borders. However, in the current world, open borders would open the floodgates for people coming here. We currently have about 50 million hispanics who have legally and illegally immigrated to the United States. Maybe that this is O.K., but how many more would come if the border were completely open? Would we say that another 50 million is O.K.? How about another 100 or 200 million?
In short, if we had open borders, people would flood here en masse until such time that the United States was no longer a more desirable a place to live than the places that these people are coming from. It would strain our resources and make the competition for jobs so great that wages would be depressed. It would import poverty and the problems associated with it. It would also change our culture.
In short, you have to set some finite limit on immigration. And what is the point of having laws if you don't enforce them?