With the esper, you can get additional Genji Gloves and Offerings. Also, the Ragnarok can only be acquired at Narshe. The other sword you are refering to is Excalibur, which can be stolen from the Goddess. It's not very powerful, anyways. Note that multiple Atma Weapons can be stolen from Sleep (the man on the Final Tier of Kefka) but it serves no use since you can't equip it, and throwing it is pretty useless as Shadow should be hitting 9999 with his Shurukiens at that point, anyways.