As pointed out daggers are in the sneak tree. The thing is it is only for ambushes. x15 damage with a sneak attack. Once you are out of sneak mode daggers will probably svck, though with stupid fast swings, poison, and enchantments they might still rock. But hey by the time you get x15 sneak attack with daggers you will also have the ability to hide in plain sight by crouching, so you probably can quickly chain sneak attacks.
Yeah... That i all knew. I'm gana find it quite the shame if daggers just blow for anything other then opening attacks. Even then... If you do the math, and assume a dagger only deals 1 less damage then our known 9 damage sword, the difference is so negligible, its not even a sword strikes difference, thanks to that +damage perk.
They would also have to be considerably faster in order stack their damage enough to be worth using over swords, because at that point your using expendable and highly depletable resources, in very rapid succession.