Most people who arrive in the USA illegal do so because they are to lazy to either fill in forms or wait 15 days to be process. It costs less to become a legal Immigrate $320 to file the correct form compared to the average cost of $2500 to get to the USA illegally.
Since people are willing to pay more to get to America illegally simply because they didn't want to fill in the paper work or wait a few days why would they fill in the paper work and wait a few days for a background check.
Would it benefit the country, you would have a lot more people going for the same low end job. There would either have to be a big surge in employment and companies or the unemployment rate would sky rocket.
Most of the available work would be in big cities and people would migrate there causing shanty towns to be set up on the outskirts, there wouldn't be enough housing. America would look like a third world country but still have Hollywood telling everyone how good it was to come.
With the high unemployment people would turn to crime, I'm guessing social systems would either be shut down or be stretch so far that they wouldn't work due to numbers. The amount of burglaries, muggings, theft’s, etc would increase and cities would become extremely dangerous.
The education system would ground to a halt with state schools closing, due to lack of funding, or having 50 - 60 pupils per teacher. People with money would send there children to private schools whilst everyone else would suffer.
Taxes would increase to try and cope with the strain in every department.
Company bosses would get richer but everyone else would suffer.
Edit ----
Michael - please look into H2A and H2B visas as you keep say there are no visa for farm workers.
Stephanie L - I'm not sure what you friend is doing wrong but if you were to look into it you would find I was correct. H2A and H2B visa do only take a maximum of a month to process and cost $320. Please look on www.uscis.gov and you should find all the information you require.
Maybe you friend should try applying for these visas, they only last 6 years but will give you the right to apply legally from a permanent resident status.
12 years is a long time and even someone who has been deported must only wait 10 years on a first offence.