Legal immigration benefits destination economies with more jobs and more spending. Taxes are also paid which benefits local, regional and national governments. Immigrant workers may take jobs away from local residents, but legal workers benefit everyone with more money put in the local economy. A greater influx of immigrant workers also means jobs are in higher demand and the economy is better off.
I am from a big city, and in fact have seen quite a few mass immigrations from countries where there were little to no jobs. I personally have seen first hand where folks have immigrated to the U.S., and have instead of adding to our economy have been more of a burden in today's economy, because they can't find a job and then get aide from our welfare programs.
I also have seen first hand immigrants that work for cash under the table, living in government subsidized housing, eating off of food stamps, and sending the cash they have earned to the country they came from.
So therefore at this time and in this economy, I believe influxes of immigrants who are looking for a job, when there aren't enough for Americans hurt our economy.