Hriston
Dungeon Master of Middle-earth
It's not always about maxing damage output, though. Sometimes it's about flavor and two swords doesn't make sense just to max damage if the character concept asks for something else.
Since it isn't about maximizing damage, as a DM, I would have no problem with you holding a dagger in your off-hand while attacking with your rapier to fulfill your character concept. Why would I? But it won't give you a bonus to your AC, and you won't get an off-hand attack with it. You're already doing 1d8 with your rapier after all!