Take communications with North Korean or other dissidents as an example. On the surface, Kim Jong needs to think we are negotiating with him without trying to support opposition and bring down his government - otherwise he may become unstable and violent. We also however need to provide moral and material support to dissidents.
Shall we just be the fools and pretend that diplomacy is not a complex game? In a perfect world, maybe there would be no need for secrecy, but to think that diplomacy does not need some level of underhandedness, deception, head-fakes, and other manipulation is terribly naive.
We don't yet know what is going to be released, and perhaps some of it very well should be public, however if any of it damages our efforts to work with dissidents in oppressed countries, or has other destructive effects on the work of the State Department, there will be serious questions about whether real-time transparency is really what we want when it comes to diplomacy.