Yeah John has all sorts of uses, particularly for this sort of thing. For instance, if you saw the person type in the first few digits of the password, you could tell John to only generate keys starting with those particular characters.

Not that you would want to do anything like that of course.

EDIT: Don't be alarmed when the window closes. When the key is found it closes the aircrack window and saves the key in the MyDocs/FAS/keys/ folder