@Starblade - Oh they do have a decapitation animation. First normal sync kill from up front when dual-wielding them is to plunge both of them into the enemy's chest. The decapitation animation I saw during the Cidhna Mine quest: Stab an enemy in the shoulder/neck to put them on their knees, then both daggers at either side of their neck and well, bring the daggers across, much like Anakin did to Dooku in Episode 3.
Also, keep in mind that daggers are usually 18 inches, or roughly 45cm in length, they can be used in normal combat as long as you're trained in their use and can dispose of the enemy fast enough. It's why marines, army, and other armed forces are deadly with knives as well as their hands. Just because you can use them to stab people in the back or jam it in the side of their neck (again, throat slitting isn't efficient as jamming it into the side of the neck), doesn't mean you can't use it as a melee weapon. In fact, if someone is very skilled and quick enough, a pair of daggers would beat a longsword purely because the longsword isn't meant to be used so close due to how it's held and swung.
Big spoiler for Dark Brotherhood relating to daggers and why you don't underestimate them as normal weapons.
The werewolf, Arnbjorn I think, nearly got killed by Cicero, same as Vazeera, the Shadowscale because they underestimated what he could do with a dagger. Sure Arnbjorn slashed up that Jester, but he was a werewolf at the time. An insane Jester with daggers held his own enough to nearly kill a werewolf and escape, though heavily wounded.