There are still some benefits that are open to legal residents but not undocumented immigrants. These include educational opportunities, many jobs (sure a cleaning service is happy to hire undocumented but a licensed daycare center, not so much), many entitlements (immigrants with fake Social Security numbers pay in but they'll never get anything out), etc.
The USA does poorly in comparison with Canada with respect to open immigration, so you'll have to clarify your "insight" as well. Knowing is half the battle: http://www.economist.com/blogs/democracyinamerica/2011/05/im...
There are still some benefits that are open to legal residents but not undocumented immigrants. These include educational opportunities, many jobs (sure a cleaning service is happy to hire undocumented but a licensed daycare center, not so much), many entitlements (immigrants with fake Social Security numbers pay in but they'll never get anything out), etc.