There's a flip side to this as well: as a voter, I don't know what information my elected leaders have when making decisions.
In theory, with a small amount of secrecy needed to make diplomacy and security function, this makes sense. The problem is that it's not actually working in practice. There are so many un-elected people who have jobs for life and also the ability to classify broad swaths of information that the incentives are all set to increasing amounts of classified data. There's no correction mechanism. In theory, you'd have Congressmen aggressively using their oversight powers to correct the system. In practice, elected officials are mostly in for life (or as long as they choose) and are easily manipulated by the system. In fact, they don't want anything to do with making tough decisions. The more things are secret, the less they have to worry with those nagging voters giving their opinions about things. I read about intelligence oversight committees being told they don't have clearance to see things and it just blows my mind: our entire system of using force rests on civilian oversight. Thousands have died because civilians have made mistakes with information they've be given -- and that's the way it is supposed to work. Somehow we've forgotten all about this critical principle.
As an example, I'll pick a topic where you guys can all call me fuzzy-headed: UFOs. I pick this topic because of its ludicrous nature. Heaven help me if I were to pick something that was diplomatically sensitive and start hammering on it. I've been studying sightings and evidence as a hobby for many years, and based on credible eye-witness testimony I'm comfortably convinced that the United States government knows a lot more about intermittent atmospheric phenomenon than they are letting on. Why? I don't know. Why not a little more openness? I don't know. What part of this information is being used by my elected officials to make decisions? Again, I don't know.
This leaves the door open for all kinds of crazy speculation. It's an insane way for a democracy to treat its citizens. Yet this is just par for the course. All I did was pick a way-out example. Laugh off my UFO example if you wish, but for every thing like that there is a thousand other things that drive public policy -- and you and I will never know about them. Taken to this level, it is a very unstable way to maintain consent of the governed.
Not only does it make it impossible for an elected official to take advice, it makes it impossible for voters to make reasoned and educated judgments about the actions of officials. Secrecy corrupts everything it touches. That's why it must be aggressively minimized.