These leaks are basically frank assessments by US representatives overseas, and they are very influential in the US's foreign policy determining process. The assessments are frank because they are not designed for public or foreign eyes - there is no need for the authors to be "diplomatic", as it were.
Now, what happens if these people get the idea that their assessments may one day be public property? Will they start to "fudge" their assessments? Make them more circumspectual? Or "diplomatic"? If that happens, surely there is a chance that US foreign policy will become misdirected - because the politicians will be making decisions based on watered down or politically correct assessements?
That’s a distinct possibility.