I'm having difficulty determining the value -resists has for naxxramas encounters as a fire mage. I see those annoying partial resists, but I'll admit I don't know enough about the partial resist mechanics to make an educated choice on how much -resist gear I should be going for at the moment if any.

Any thoughts?

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http://forums.elitistjerks.com/viewtopic.php?pid=169010

Level 63 mobs act like they have 20 resists (5% resist rate) against non-binary spells. Can not be overcome.

-resists is solely a PvP stat.

From reading those threads it looks like it isn't that -resist is purely pvp, but that it merely can't get beyond that 5% on a 63 mob. If I'm reading correctly, particular mobs do still have high resistances sometimes (Lucifron supposedly is high SR, Golemagg has high FR) which can be overcome until that 5% via resist gear.

Correct me if I'm wrong of course.

Technically, you're correct. Various mobs in BWL and MC have high fire or shadow resists. I haven't heard of any in Naxx or AQ40. Keep in mind that every mob there will be at -85 or more resists for most mages from CoE and Arcane Focus.

It's like Weapon Skill and Glancing Blows. No matter what, non-binary spells will be partially resisted on higher level mobs.