It's a big deal because it's almost impossible to use this information in a way that won't have a huge cost to
1) the people visiting the US and
2) the US public (via tax support for enforcement which is driven by this policy)
Wolfram Alpha says that 322 people visit the US in a year.
Let's assume the technologists in charge of this (who are really no different than any other corporate engineer - capable, but fairly disinterested in their work and apathetic/unaffected by the results) manage to make the process 99.9999% effective in screening out the innocent, and 100% effective in screening terrorists who enter the US.
Well, that world-class accuracy gives you 99.9999 * 322 million => 322 people a year who are caught in the dragnet
(and in all likelihood, it will be multiples of this number; likely thousands of individuals).
So now we are interrogating, investigating, and suspecting 322 innocent people in order to catch the... oh, wait; there really haven't been any terrorist attacks like 9/11 since... 9/11.
The Boston Bombers were home-grown. Anti-abortion extremists, religious fundamentalists, bullied schoolchildren, and paranoid schizophrenics have all tried or been accused of trying to wage terrorism on the US, but there's not a single event[1] that it looks like this policy could have prevented. But we're investigating 322 individuals (and their families, friends, and close contacts) in order to do it.
And at what cost? We will never know, because those numbers are classified due to their sensitive nature.
In the end, none of this matters: the government was never interested in terror prevention in the first place. They just needed a new bogey man to replace drugs, the Russians, whatever as a reason to ignore the constitution.
The war on terrorism is a farce.
1. https://en.wikipedia.org/wiki/Terrorism_in_the_United_States...
* edit: added the parenthetical note on numbers, multiples of estimate