You guys are referring to SIPRNET. Its highest classification is SECRET (hence the S in SIPR). Traffic classified higher up (TS, SCI, RD, etc.) all goes through JWICS.
Manning leaked stuff from JWICS, too.
It seems like your estimation of those events was shaped solely by your interpretation of what happened in the leaked video.
The question about secrecy vs. privacy is: why stop at foreign affairs? If you support full on transparency, then you're also against secret voting by representatives, and it's fairly easy to see that either position in this case has its drawbacks.
I'm sure there are things that are cloaked in diplomatic secrecy which would benefit the world if they were revealed. However, in general, diplomacy is a good thing. It ends wars, stops them before they begin, and even when you're dealing with a bad nation that wants to do bad things, it is always a good thing to be able to work diplomatically with them. That's why a blanket dump of diplomatic documents is a bad thing. It impedes diplomacy, which is not a good thing. You can argue that blanket dumps of things like the Iraq and Afghanistan war, by hurting the war effort, are a good thing because the war effort is not a good thing. However, that argument fails when it comes to diplomacy. Simply pointing to things and saying "this, releasing this is good" doesn't justify the notion of a general document dump.
Take the Iran situation. It doesn't really even matter what you think the best result is: Iran with nukes, Iran without nukes: a diplomatic solution is better than not having one. It's not a question of who is Right and who is Good: it's a question of taking the states vitriolically opposed to a nuclear Iran, Iran, and Iranian allies, getting them to the table, and emerging with a solution that doesn't involve war. Making all aspects of those negotiations public empowers the pro-war crowd in the US, for example: you hear the unabashed vitriol some parts of the US have for the negotiations with North Korea to bribe them into giving up nukes. Doing these sort of negotiations publicly would not promote a peaceful solution: it promotes Iranian chestbeating about their inalienable rights to nukes, Israeli posturing that they will never allow Iran to go nuclear, and the various factions within the US government that are aligned with one party or the other to take a hard-line stance. None of that helps a peaceful solution. Any good solution requires a number of nations to assist, who just got quite embarrassed by their private fears being leaked and who will be less likely to assist in the process as a result.