There isn't a practical reason to use a single weapon in NWN because the other methods of weapon-wielding (Dual, Shield, 2H) are all just statistically better.
In Baldur's Gate, there was a proficiency for characters that used a single weapon, which if I remember correctly was both an AC bonus and a bonus to crit range.
In the slightly different yet still "faithful" Pillars of Eternity adaptation, using a single one-handed weapon conferred an accuracy (AB) bonus to attacks.
Given the choice of the two options, I'd say that the latter makes more sense from a balance and gameplay standpoint - Possibly with a "scaling" bonus with level (Fighter level specifically?).
One-handed bonus(es?)
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