I think they are. They can't fight as well as fighters or black belts, and all the black magic they do learn they can't use very well. The only real benefit of them is that they can still kind of fight, and can help out when hasting the melee classes. That's the only use I've ever found for them. Not to mention that while they're still thieves they basically can't do anything. I would personally rather have either a second fighter (to absorb damage and help dish it out) or a black belt (to dish it out hardcore) in place of a ninja, or maybe even a red mage (they have cool hats). I just don't see a use for ninjas at all.
So, are they useless or not? Discuss.