Earlier quoted context omitted.
Consider this: How many (sober) politicians will now have an open heart discussion with an US diplomatic official, which really couldn't reach his own voters/subjects? Without being that knowledgeable about diplomacy internals, that should be worse than most any scandal.
If politicians no longer feel comfortable stating opinions or plans which would be unacceptable to the electorate and/or their own voters, I for one don't feel that's entirely bad. I'm pretty much in favour of politicians being open about their plans and principles.
The end result of the Wikileaks cable dump is that it's going to make it much more difficult for American diplomats to engage in calm, rational diplomacy behind closed doors with most of the world's countries. That may bring immeasurable harm to America's foreign policy and make it harder to maintain a peaceful and stable international system.
I don't believe this leak was necessary or in the best interest of most of the world's people.