In fact, the reinstatement of border controls seems to be an example of “security theater” – a policy intended to make the public feel like something is being done. But, far from making Europeans safer, rolling back Schengen would actually hinder the fight against terrorism, because countries would be forced to devote valuable resources – thousands of police officers, if the agreement were to be abolished altogether – to checking documents at borders. Those resources would no longer contribute directly to investigations into terrorist activities.

And those investigations need all the help they can get. After all, the objective – to identify a few terrorists hiding among millions of law-abiding citizens, before they commit a violent act – is the equivalent of finding a needle in a haystack.

Source: https://www.project-syndicate.org/commentary/schengen-open-borders-help-security-by-daniel-gros-2015-12