Yes, DHS should use crowd-scanning facial recognition technology, because it would allow them to target people who are known to be threats. If DHS could have used that technology, it might have made a difference in the Boston Bombing. The practice allows DHS to focus on the problem people, and leave everyone else alone.
Facial recognition scanning technologies work wonders for police in tv shows, however putting this technology into the hands of the DHS will more likely cause them to use it not on terrorists or risks to the American people, but on the general public such as petty enemies of the United States like protesters or people who speak out with opposing ideals. It's another step toward an Orwellian dystopia.