Actually, normally, it doens't reduce the actual chance, but it does help reducing damage/effect further and reduces the duration. (And for fire/shock, it makes it so the 'ticks' happen less often) For fire, some attacks will stop causing it once you reach a certain amount of resistance, although for other status effects it simply reduces duration and the effect. (For example, stun resist will make stun slow you down less than normal, while weakness transforms you into a turtle)

In the Vanaduke example, it's worth using a shield with fire resistance, because only then you can become completely immune to his fire streams. (Unless you have something like two maximum fire resistance variants on your armor) Elemental defense doens't really help as much as actual fire resistance.

But in general, status effect resistances need to be reworked - Status effects in themselves are too strong, and the resistances appear to stop at a certain point (I think with the resistance update on shields, simply having one shield with maximum resistance equipped already reaches such point? If not, a single five-star armor piece with the resistance will do it, or two lower star ones), after such point resistances either have really minimal effects or no effects at all. (Fire, for example - Minimal time fire will last is 4 to 5 seconds, and using Vog Cub set with Volcanic plate shield haves me take 1 to 1.5 damage from each fire 'tick', while without the shield I take 1.5 to 2.0. This is on T3 - Other tiers don't end up making much, if any, difference. Actual chance and duration are not changed, and only two or three rare fire-inducing attacks end up being prevented when using shield)