I don't think they'd ever make the argument that the content itself is dangerous - but it may give them more information about you, and let them decide that you are dangerous.

At an extreme - if you had a document plan_to_takedown_flight_123ABC.pdf or diary_about_my_drug_smuggling_operation.txt, whether or not the data itself is a threat, it could indicate that you are.

I mean, I think it's bullshit, the same way I think most security screening and border guard questioning is bullshit, but if you wanted to maximize your border security (and keep out, ahem, "undesirables") and don't care about people's privacy, it's perfectly logical.