People tend to make and disseminate plans to their minions prior to executing them. Thus, the plan could be leaked before the thing that is planned occurs.
Oh, wait. The Pentagon already does this: The have battle/attack plans for every country in the world! So I guess they should leak those and let people scream in outrage?
A plan is not equal to intent. And you only know intent after it happens, not before.
By the time a schedule like this is announced (even internally in the military), it's already in progress!
Not that this is a problem inherent to Wikileaks; you could use any service to communicate the same thing. I'm just giving an example of a time where publishing sensitive documents could be done ahead of time to disastrous consequences.