When the art team at Blizzard developed the class icons for World of Warcraft, it based them on that class' weapon of choice. Mages cast spells through their crystal-laden staves, druids fight with their paws and sharp teeth, and paladins wield a mighty, righteous hammer.
The rogue class has been represented by the swift, poisoned dagger for years. While our dark leather armor and our brittle yellow class text are easily recognizable symbols of the rogue, the dagger is our one true sigil. While only two of our abilities can't be used with swords or other weapons, both assassination and subtlety rogues are still forced to bend the knee to the dagger god. Even the most stalwart combat rogues carry a dagger in one hand. Why not go all the way?