A leak is necessary when it demonstrates the government or the leaders are lying. There's zero democracy when the citizens aren't told what's going on.

But this is not the only information that wikileaks is leaking, so they seem to ascribe to another definition.

Not really, no.

Everything Wikileaks has leaked has been something that shouldn't have been secret.

In some cases we wish it would have stayed secret, but that's our fault (collectively) for breaking our own laws. For instance, had we properly investigated the behavior of our troops in Iraq there wouldn't have been a need to blow the whistle on the situation.