Depends per subject and intelligence service and enemy (there is more than one).
> I don't think secrecy is important in national security anymore.
Why?
Think of it this way. I'm Dutch. Our secret services hacked into the GRU's camera system. This allowed attribution. Our secret services also did the physical last mile infiltration of Stuxnet.
Did I, as random Dutch citizen have to know this? No. As the enemy does not know it, and will know it when the Dutch population knows. These are two examples where a need to know basis makes sense, and is very limited.
By contrast, with something like a secret mass surveillance program, there is no "end" where you keep something a secret for 18 months but then everybody finds out about it through official channels, so things like that don't get to be a secret. One way or another the public needs to timely find out what's being done in their name.
Indeed, but the enemy usually has the intelligence services to know it anyway, leaving your citizens uninformed as the only real side-effect.
I don't think secrecy is important in national security anymore. On the other hand, the secrecy is used for evil, such as keeping citizens in the dark around government plans to control popular opinion in their favor, falsely positioning themselves as the people's superiors.