Barriers to entering Mexicans simply encourage them to stay in this country longer, and make them less likely to return home for long visits. On net, the number of illegal immigrants appears to go up. For more, read the always stimulating Virginia Postrel.
My take: I know some of these people (the Mexican immigrants), and I can tell you, this is absolutely true. Given border crackdowns, they make a conscious calculation to stay longer in the United States, rather than migrating seasonally.