1) As we've seen with Wikileaks "airing" is exactly what these leak sites amount to - hot air. Malfeasance in government has to be known about to be fixed, but merely being known is insufficient. There have to be robust accountable bodies in place to tackle the corruption.
2) Most free states have parliamentary committees of elected persons, appointed legally to deal with leaks.
3) The public press handle leaked material all the time, and generally pursue the story, not just air it.
What if 2 and 3 fail? Then you're ****ed sideways anyway. At that point the engine of government is so badly twisted that leaking material won't help.
Internal oversight is nice when it works. It doesn't all the time. The press can be more worried about the bottom line, lawsuits, etc. and often times don't feel particularly adventurous about being the first to stick their necks out. Ideally whistle blowing entities like Wikileaks hold themselves to a high standard and consider whether the material is in the public interest and in doing so build up a reputation for integrity. It's not a perfect word.