--> My WOTLK-version of this WA, which can also switch talents, can be found HERE.
This WA makes it possible to automatically switch between Equipment Sets by hovering over, targeting and/or killing NPCs in Naxxramas, Ahn'Qiraj (Temple & Ruins), Blackwing Lair, Molten Core, Onyxia's Lair and Zul'Gurub (…Hitcap/Trash/Undead Cleansing/Resistance/Riding Gear - you name it) by utilizing the AddOn ItemRack Classic (see Curseforge/Twitch). It works with all game client languages.
Specifically, it can equip pre-defined sets of gear…
1. as you enter raid instances
2. as you mouseover or target and kill bosses
3. as you mouseover or target and kill Level 63 trash mobs
4. as you mouseover or target and kill any further NPC that you define under "Custom Switch Options"
…in ZG, AQ20, ONY, MC, BWL, AQ40 & NAXX.
By enabling Adaptive Sets in Custom Options, this WA can also automatically equip further sets of gear depending on…
a) if a boss is Undead or not and/or
b) how many players of your class are in your raid and/or
c) if you are worldbuffed (Furthermore, you can also define thresholds for stats like spell power or +heal for your character to be regarded as 'worldbuffed').
--> With this feature, simulated BiS lists (like this one or this one, for example) can be automized completely. However, setting up gear sets for Undead/nonUndead targets for two boss fight lengths should already cover most BiS scenarios.
Switching is very useful, as "optimal gear" usually differs greatly between individual Bosses and Trash. Especially for Trash, you can switch out +Hit for far more valuable bonuses. In Nax, you also have multiple occasions where special sets of gear against undead targets make a lot of sense. Healers may also benefit from exchanging +Heal/MP5 sets. Perfect equipment switches create an advantage in speed runs and parse-oriented gameplay.
--> My personal "ideal" set config for Mages is here.
--> Feel free to also check out my entire UI here --> Zippy UI for Classic (Mage/DPS)