Ever tell someone you liked their food when you didn't? That what they were wearing didn't make them look like an idiot? That you were listening when you weren't, or laughed at a joke when it really wasn't funny?
If you thought, "yes," you've engaged in an act of personal finesse that, on a much larger scale, with much higher stakes, involving the monumental egos of statesmen and national leaders, is part of what diplomats do every day. Most workplaces couldn't get by without candid private communication, whether we're talking the kitchen of a restaurant or a corporate office. American foreign policy can't function without it. The new Wikileaks revelations will have a chilling effect, not just on what American personnel tell each other, but what they are told.
When Wikileaks unveiled the Afghanistan war logs, it said it was revealing the reality of a war American politicians had hidden from view. When it revealed its Iraq War logs, it shed light on the American government ignoring human-rights obligations in the name of stability. The irony of its large-scale document dump of American diplomatic cables is that it undermines American efforts to bring international conflicts to peaceful resolutions.
I'm not against leaks as a whole -- when leakers are exposing some kind of government malfeasance that would otherwise go unreported, leakers are doing something brave and heroic. But as Matthew Yglesias writes, part of the government often acting arbitrarily in declaring something secret is that just because something is secret doesn't necessarily mean it's nefarious and needs to be exposed. To the extent that there are matters worth bringing to light in the latest Wikileaks documents, I don't doubt they could have been addressed without the collateral damage.