But if you didn't have the KC, you would already have the next card in your deck instead, so it wouldn't really make a difference...
If you only have one action left, it is better to play the KC than not, which I guess is probably the point being made, but it isn't better than not having it. If you have extra actions, it is almost certainly better to play the Library on it's own and hope you draw something better to use the KC on.