Although it wasn't with a zerker, my first lesson of resists was after I was soloing Varsoon and his 'ladyfriend' as a L44 swashy. Using offensive stance for DPS and interrupts I was taking too much damage. In defensive stance I would win most times, using a good poison and a heal potion.Then I got ACT and parsed Varsoon. A big chunk of his damage was heat. I had like 12% heat mitigation. A quick refit later, I had 4 new jewelry/ranged slot items with +heat to swap in, and I could sit at over 50% heat mit, with a reduction in other stats. But the difference was night and day, and I was basically winning with over half health every time, never any real risk at all.So I took that lesson over to my L70 pally, and I now make a note of damage that nameds in instances do, and where appropriate swap in armour here and there. There are some jewelry items with +1000 resists on them, many with +800 or so. Against the right mobs they help a lot
Those Light Chitin Rings are nice not only for phys mit, but across the board spell mit as well.Ultimately it's down to how close the fight is. If you're pushing the envelope so to speak, it can be very useful to pay careful attention to resists. In general situations, it probably doesn't matter.