So one of the ways of collecting intelligence is knowing who is who. Who they really are, their past etc. If identity can be fluid and hidden then it's hard to know what people's intentions are
German intelligence and police have already highlighted they don't have the faintest idea of who is in the country, or where they are, people come in, may or may not get processed - and then dissapear.
I also think its important to point out this man so far seems NOT to have been a refugee from Syria at all but from Pakistan.
On ITV this morning they were confusing Syrian refugees with migrants from all over - which is a big massive issue! No wonder everyone is confused about it all 
I feel sorry for the EU in many ways - because FOM is a noble idea, there is no denying that. But its come at the wrong time in history, and enabled crime, criminals, people trafficking, gun running and an ISIS terrorists dream.
No day of the Jackal - disguises - hiding guns, plots and plans, you can go where you want and do what you like.