The leaks don't discredit diplomacy, they simply reveal its normally hidden workings, which are a bit rough around the edges. Not all diplomats are particularly diplomatic.
Ultimately, though, the leaks give me more confidence in diplomacy, because they show that even authoritarian, closed governments like that of China have a solid underpinning of common sense. The cables reveal that China is not nearly as friendly towards North Korea as their Communist affiliation forces them to pretend; in private, they concede that Kim Jong Ill is nuts. The cables also reveal that many middle eastern leaders are just as concerned as we are about the prospect of Iran becoming a nuclear power.