Anyone can use Stabby or Slashy Weapons as far as I know with almost any Pirate. The bonuses that are applied however deal with the talents that help those weapons. A Witchdoctor uses Will where Stabby and Slashy weapons use Agility and Strength.
As long as the Spells show up in the Powers Bar during combat, they can be cast unless they are grayed out (this means that for some reason the Pirate can not cast or use them). For example, I had a Sneak Attack Power but, because I had not equipped a melee weapon, it was grayed out and I could not use it.