Daggers are a tool more than a weapon; They are a weapon for non-weapon users. That might seem like a nonsensical statement, but it's true. In a knife fight, it's better to bring a gun. There are two specific roles in which daggers shine - and they shine to the most overpwoered degree.
The first is that of the rogue, assassin or sniper. The x15 damage modifier perk in the stealth tree is strong enough that a purist rogue can deal fatal damage without taking ANY perks in the one-handed tree; if you go full on stealth and take shadow warrior, you can even fight straight up fights 5 on 1 and come out trumps by forcing impromptu sneak attacks.
The second is that of the artificer or enchanter. An enchantment is an enchantment, regardless whether it's a sword, dagger or warhammer, if you have a strong weapon enchantment on each dagger, then you can lay down a devestating flurry of enchantment procs that'll annhilate your foe in a storm of spellfire. The dagger's low damage is of little relevance.
So, as I said - the dagger is the weapon of choice for those who do not use weapons; but instead use enchantments, poisons or stealth
Have a look at this shadow warrior in action and be inspired

http://www.youtube.com/watch?v=KETNiqHg0oM