It is a common misconception that Garniture of Interference, Surcoat of Counterpoise or Boots of Polarity would increase the resistance secondary skill in a similar way that for example Bow of Elven Cherrywood increases Archery skill. However, this is not the case. The "resistance artifacts" increase the magic resistance of creatures in a similar way Resistance secondary skill does. Basically this means, that wearing any of those artifacts gives the magic resistance bonus with or without the resistance skill – unlike with archery artifacts and Archery skill.
While resistance may not be reliable, it can turn the tide of battle against a magic user when it takes effect.