Basically, the defense of one-handed weapons only matters if happens to you to use an one-handed without any shield. After you equip a shield, you will have the defense stats of these weapons ignored, and the defense of the shield will compose the overall defense value of your char.
However, if a weapon has for example a plus one on it's defense stat (for example, like the Bright Sword with +1 on def), it means that even when you get a shield, your global defense will have an extra point on it! But if you get a one-handed weapon with a subtraction after the def value (like the -2 present on the Assassin Dagger) you will have the subtraction of two points on your global defense value! So choose wisely these weapons... Having less defense and more attack with a subtractive weapon may be or may be not an advantage depending of the situation presented in-game!
Another curious thing to point out regarding one-handed weapons: The def value of one-handed is extremely good for mages, since they will benefit with item with relative low weight and bonus defense, as the Bright Sword already mentioned, being excellent if they already got a decent shield or spellbook to hunt (keep in mind that the Bright Sword is an item not restricted only to knights). Mages may have the option to imbue one-handed weapons, making a hunt even more profitable!