Like Stabbey and VC said, Warfare would be a better investment than the Dual-Wielding ability since Warfare increase ALL physical damage by 5% while Dual-Wielding only increases weapon damage. In other words, Warfare will improve your Necromancer damage skills, while Dual-Wielding won't.
Weapon combat abilities also improve damage of skills that are based on weapons.
For example bonus in single weapons will increase base attack damage, but also damage from Crippling Blow, from example. But not Tentacle Lash or Shield Bash.
For example, for Archer, I would always first focus on Ranged weapon ability, before Warfare, since pretty much all damage skills for Huntsmen are weapon based. And ranged gives both damage and critical bonus, unlike Warfare.
