I don't think there is a good argument for illegal immigration. What you're basically asking are reasons why it's okay to break the law. You could say that they take the jobs that American's don't want but that's not true. Recently, a restaurant in AZ was busted for having illegals working for them. The illegals were arrested and hundreds of law abiding Americans showed up to apply for the vacant jobs. That tells me that most Americans are willing to work if they can find a job.
Its a bad thing to think that immigrants aren't good. some people are heroes and they might be immigrants. also ispanics work hard. Either way u shouldn't gudge them because they may become your boss! :)