Well, it was originally created for language learning. With vocabulary there really is so much of it that you're better off learning the stuff that sticks and moving on from the handful of items that just don't. In some other fields you really do need to memorise the whole set of items, but that's just never going to happen for second language vocab.

The other thing about leeches is that they're a sign that you probably want to try to change something about the way you're trying to learn them, because what you're doing now is obviously not working for those things.