This weapon is the most forgotten of all weaponry in FF4, due to its natural inferior stats to the Legend Sword. Additionally, there are only 2 Spirit monsters after this point in the game to take advantage of this... which doesn't make this weapon seem worthy of a look.
It still has an intricate use for those that want to find something useful for it...
1) Allowing Tellah and other frail units like Yang (and Cid's uber tanking) to take less damage. Curse status isn't a great stat, but reducing the monster's attack power by half is always a good start.
2) Flan/Pudding killing is thought of to be a Drain weaponry special, which is generally the case. However, the Ancient Sword allows you do this one step earlier... as in conjunction with Yang (equipped with an elemental claw), you can make Yang much more effective w/o having him use Build Up/Power and kill Flans easier by leading off with the Ancient Sword and reducing the Flan/Pudding's monsters armor stat to something more reasonable... and allowing the elemental multiplier for Yang's claw to do the other half of the dirty work.
Anyways, there aren't any other monsters that fall into the overpowering attack catagory to make Curse status even more useful, but that's where you can find for this weapon's limited usefulness.
We have EVEN MORE themes now, but some old ones are gone...