Double Saber is a Fighter's primary armament. It's your single-target destroyer with Deadly Archer, and the gear provides you with some AoE. As a FI, I probably use my Double Saber 70% of the time, possibly more.

Daggers have two uses: Killing flying enemies who are otherwise difficult for melee to reach, and striking elevated boss weakpoints which are otherwise difficult for melee to reach. They are your aerial armament.

Aside from the ubiquitous knuckle-dash, knuckles are good for dealing heavy damage to slow or stationary single targets like Vardha's core (using Flash Thousand) or for linear AoE (charged Straight Charge). They are also a utility weapon in that they provide you a convenient stunning attack (Quake Howling) and a convenient pop-up attack (Slide Uppercut) which are useful for dealing with certain obnoxious enemies (Krabahda, I choose you!) or for bailing out a comrade who has a temporary brainless moment and gets grabbed by Gwanahda (Gwan is stunnable when he grabs someone).

Of course, for the linear AoE purpose, I generally prefer to just go FI/HU and carry a Lambda Aristin for Sonic Arrow spam.