Like Draupne said, they aren't supposed to be big game changers. And how are they weak? If used correctly you get to scout your enemy from a safe distance without them knowing. As long as you use that passive correctly that is. And if you don't see that as useful, what about being to able to see invisible units? You can catch a squad of assassins while they windwalk towards your hero, meanwhile the other races will be caught off guard.