All too often, secrecy is used to cover up policy and procurement cock-ups. The public, whose taxes are being used to pay for such cock-ups do have a right to know how their money is being spent, and more particularly have a legitimate interest in whether their money is being spent wisely. Moreover, in the long run, it is in the interests of the services to be as transparent as is possible (within the obvious constraints imposed by security relating to ongoing operations) if they are to maintain public support and goodwill.
There is a culture of secrecy which is profoundly unhelpful to the best interests of the forces. The question should not be "what is the absolute minimum that we can reveal to the press/public" but "what NEEDS to be kept secret for military purposes". Preventing the embarrassment of politicians, civil servants or senior officers is not a good reason for secrecy, and indeed helps in the perpetuation of many of the problems which now bedevil the forces. Half the reason that BAE can get away with fkn up in spades on (say) MRA4 is that so many previous disasters (Foxhunter, for example) were swept under the carpet.
I'm not sure which 'vanished thread' you're referring to, BEags, but would say that anything relating to the V-Force (which vanished 22 years ago) or indeed to the nuclear deterrent (already a memory) ought to be something that could (and even should) be talked about. It's simply not necessary to keep the Bouncing Bomb secret into the 1960s, as we did, and nor is it necessary now to have a screaming fit if someone talks about how, when and why we might have used WE177 (say). It's about as relevant as good archery tactics...........
And what would or would not have happened before D-Day is not relevant today. Society has changed, and whether you like it or not, the public demands greater accountability from those who claim to act in its name - whether they be Politicians, Civil Servants, or members of the armed forces. Public consent to be kept in the dark no longer exists, and the public no longer believes that politicians and generals 'know best' nor does it trust them to act without self interest.
Today, unfortunately, the attitude is that "if you want my money for new toys, pay reviews, etc. then you're going to have to tell me what my money's being spent on, you're going to have to prove that it's being spent wisely and I'm going to need convincing that it's not going on equipment programmes whose costs spiral out of control, or which isn't fit for purpose when it arrives, or that equipment that I have bought and paid for isn't being jettisoned prematurely." And you're going to have to live with that, just as the public will have to live with not knowing details about tactics, parametrics of particular weapons systems, etc.