reading alot of the message almost everybody said that thieves are useless unless they are dualed or multi class.

i have an assasin thief and i think he's great.

They are the best single class as they improve their backstab beyond x5. It's not thieves are weak, its just they are great for dualing to other classes to add flexibility, especially mages.

Yes, it is not that a normal thief is not useful, it is just that fighter/thief is a lot better that's all.

You can only advance and specialize so much in a single class, like El_kalkylus mentioned, a fighter/thief offers more/better opportunities.