‘Illegal immigration is to immigration what shoplifting is to shopping’ - Link
There is video discussion at the link.
Simply put, we need to have immigration rules. For them to mean anything, we need to ensure that the rules are respected and to ensure that, they need to be enforced.
The alternative is the situation that we have now. There is no fear of breaking immigration law. There is little fear of being caught and even if they are caught, there are many examples of people being set free to continue living (and working) here illegally.
Anyway, if the country is so willing to legalize illegal aliens, why not just annex Mexico. Then we can offer citizenship to all Mexicans, not just those who decided to violate our immigration laws.