If you synth a 'Save the Queen', you can say "Hey! Everybody! Notice me! I've got a 'Save the Queen'! Aren't I something?"

You can also get Amarant to throw it at opponents to cause a few thousand points of damage. Only Beatrix can equip Save the Queen. Since she already has one and she can't joint the party permanently anyway, it's not an entirely useful sword in the long run.

I think that it adds a little bit extra to the ending, though, in the same way that the Hammer does. Cinna's Hammer is a useless weapon, but it's necessary in order to synth the mighty 'Tin Armour', and having it in your inventory at the end allows you to see an extra few seconds of conversation during the ending sequence.