I read Derpderpderp's comment to be referring to the existence of plans, not the specifics. The existence of the Normandy invasion plan was hardly a secret. It was unavoidable that the Germans would be able to tell a major offensive was being prepared. And one very clear candidate was for the Allies to take the more direct route and hop across the channel. The exact specifics (where paratroopers were being deployed and when, which beeches were to be hit etc) are important secrets and my reading of his comment is that they should remain secrets. But the actual existence of the plan was obvious and should not be secret.

The militarily valuable secret is that the Normandy plan was that that was the one the Allies were counting on. There were other plans, for example the plans for the invasion of Sicily surreptitiously delivered into German hands by Operation Mincemeat. Or plan to attack from Norway using Edinburgh as a staging post as part of Operation Fortitude was another candidate. But the existence of the plan is the

You do lose some military information. If the enemy knows you have the capability to track someones movements based on his phone number, they are likely to take mitigating measures. But without revealing this capability, its impossible to have a democratic debate on the appropriate safeguards necessary.