They added a whole new weapons for a mere change in damage. Sure, it's balanced, but it's also pointless to add a whole new weapon. Just add a weapon mod that does the same thing and let players customize their weapons looks a bit and it's a lot better. But that would require too much thinking, so it's a much easier and cheaper way to just add a unique weapons there.
They add variety. I don't know as much about unique weapons in other game, but in New Vegas they don't just have a higher damage.
Comparing Dinner Bell to an unmodded Hunting Shotgun: Dinner Bell has greater damage, less spread, slightly higher crit damage and 50 less health. I'd actually say it's one of the less useful unique weapons.
Comparing the CZ57 Avenger to the unmodded Minigun: The Avenger has alightly higher damage, higher RoF, nearly half the spread, slightly higher crit damage, weighs 7 lbs less, costs 5 AP less to fire and has 200 more HP, but its magazine is half the size.
This is what I personally love about the unique weapons of New Vegas, its because they are truly unique. They aren't all equally better then their regular counter parts.