the U.S. is also negatively affected

There are significant adverse economic impacts to the U.S. when Lawful Permanent Residents are separated from their families.

Immigrants with immediate families abroad often remit part of their earnings to them.  Every year, an estimated 4.2 billion U.S. dollars is sent out of the country by LPRs who are supporting their spouses and families.1  Family unification would result in this money remaining in the U.S.,  as families of LPRs that come to the country add to the consumer base.  Money that would be sent abroad to support them is now spent in the U.S. 

Though it can't be generalized, over time, entrepreneurial spirit in immigrants has resulted in a lot of success stories.2  Immigrants have a proven track record of creating jobs.  An LPR forced to choose between country and family may choose to leave the U.S.  When an LPR leaves the country, they take their jobs with them.  Jobs that may have been created in the U.S. as a result of the LPR's entrepreneurship would be moved elsewhere.  This is demonstrated by a case study on the impact of reverse brain drain on the Taiwanese economy.

In general, LPRs contribute significantly to the success of the U.S. economy, as, on average, immigrants contribute $80K more in taxes over their lifetime than they receive in benefits.3 

Taking aside moral aspects of this issue, the U.S. has still much to gain by uniting families of LPRs.