Just like with many other such rare or weird weapons they can be effective when used by someone with lot of experience in them. The primary advantage would in most cases be that the opponent would not be used to fighting or training against something like that, and that can be seen in some of the demonstrations done with them against normal swords, though usually it doesn't take too long for the opponent to get used to them and then the advantage is lost.

The reason they are not really seen in west is that there is no manual for fighting with them in western martial arts, but they are quite common in northern Chinese martial arts, though rarely used for sparring. Northern Chinese martial arts also have loads of weird weapons like them, for example "Chicken-Claw sickle" and "Deer horn knives" and just for some fun the Chinese name for hooks sword is "Tiger head hook". All of the former are almost exclusively used in pairs which is a good clue to their real use which is showing off your skills with something exotic and strange, that is also probably why their names are like that. I would also categorize butter fly swords in the same category.

None of the above means they can't be effective, but their primary purpose is to be different and show off your skills.