Well, there is a bit of overlap between some of them, but for the most part, you just get a hell of a lot of skills. I don't think they're useless. And some of the most useful ones are monster classes that a lot of people don't even bother with.
To make it a little extra complicated, there are hybrid skills you can learn by mastering two classes in a sequence. So for example, if you master the warrior class and then the dancer class, you get the really useful skill "sword dance". Then if you want to get the "Rob Dance" hybrid skill you can master the thief class next!
Dang maybe il