"Someone has slain a wealthy merchant and left him here in the path with a Longsword still in his back. You remove the sword, examine the plain weapon. Nothing to indicate who might be responsible, but the sword is still true and strong so you keep it. As for the murder, the trail is cold. There is nothing you can do."
Even though the bonus is insubstantial, it is better than nothing. For a small price, the Longsword is a good deal, but there is no need to keep it after better weapons are found.