When I first joined ALIPAC, I came across a post where I brought up the argument for allowing legal immigration due to the future "labor shortage" since it came up in my Organizational Behavior class. Since then I've done the reading, and this is what is says

The U.S. labor shortage is a function of two factors--birth rates and labor participation rates. From the late 1960s through the late 1980s, American employers benefited from the large number of Baby Boomers (those born between 1946 and 1965) entering the workforce. Specifically, there are 76 million Baby Boomers in the workforce. But there are 10 million fewer Gen-Xers to replace them when they retire. The problem becomes serious beginning between 2007 and 2010, when the major exodus of Boomers from the workplace begins. Importantly, in spite of continued increases in immigration, new entrants to the workforce from foreign countries will not do much to correct the supply shortage.
And further...

In times of labor shortage, good wages and benefits aren't going to be enough to get and keep skiled employees. Managers will need sophisticated recruitment and retention strategies. (p. 20-21)

Source: Robbins, Stephen P. Organizational Behavior, 11th Edition. New Jersey: Prentice Hall, 2005.