I'm mainly into roleplaying, so I could always only use a dagger for the DB quests, but that wouldn't be nearly as cool as when I'd actively use one-handed as my main offensive skill. I'd have to hack 'n slash them to death instead of the more 'realistic' slash to the throat - done. I do want to keep him as a pure mage no matter what 'though, as I already have another character who uses one-handed (and she's no Dark Brotherhood material at all).