“I do this for good reasons, trust me” is not a valid argument.

Yes. The problem is, when one country has had a intelligence agency and the other has not, the one with the agency has a advantage. At least, under the same conditions.

I see the tension between a republican (res publica, “thing of the public”) State and the existence of such secrets. The question is if a state without this could exist under the current circumstances. There are a lot room for doubts here, I fear.