I suspect the results wouldn't be worth the effort. Unless they're very scrupulous about model matching you can wind up with minor variance which can vastly impact a fight.

For instance, don't underestimate how important a minor difference in hitbox size can be. A tiny hit box on a boss that chains or AOEs (KT blocks, Thorim Lightning, Beasts Paralytic Toxin) can vastly inflate the difficulty of some mechanics, while a huge hit box is "easymode." Conversely a huge hit box can run into parry issues (melee is not actually behind the boss) and range to boss-centered AOE abilities (Gormokk stomp).

Testing something with hitbox size X and having it go live with size Y can make a stiff fight utter faceroll or a simple fight exceedingly difficult. Doubtless there are other minor effects which could go astray if the model isn't properly tested.

Of course they could test real models internally and dummy models on PTR, but how long before a real model made it in the PTR build by human error and all their effort is subsequently wasted.

It used to be that a patch would go live and people would experience brand new bosses with the learning curve involved. It could take weeks for tactics to develop and spread to guilds that weren't bleeding edge. Now with PTR testing everyone can go into battle armed with this foreknowledge. Knowing this Blizzard can up the difficulty of fights, thus producing better bosses. They're also able to do the latest thing of limited attempts. Going in with zero knowledge you could spend attempts just reaching phase 2, suddenly have new abilities with which to learn and deal, wasting more attempts. Your average (non-top-100) guild could even waste all its attempts before it fully understood all a boss could do, making it hard to accurately tailor their gameplan.

Although, to be honest, all these superpowered paladins just keep piling up...

I've been wondering about what Bloo Driver pointed myself. And although I know it's pretty much inevitable, this being Arthas, I'm secretly hoping that the last fight doesn't at some point turn into a parade of victims' ghosts haunting Arthas and him going "Get off my head, I have killing chores to perform!" Also, I've given up understanding what exactly is Arthas' role inside the Lich King. Or the scope of his powers. Or what happened to Ner'zhul. I fancy meself some secrets and enigmas, but somehow I fear these questions might not even get addressed.

And, most of all, my Slaughter Sense is tingling...I can't see any of the major characters dying at Icecrown - not counting Bolvar's eventual sacrifice, or Saurfang Jr's "release". I may come across as bloodthirsty regarding characters, but I just think you can't have a proper final confrontation with the main "god-like" (really?) villain without him showing he's "NOT" kidding, however much he laughs at your feeble attempts to dent his armor or at your king/overlord/archmage/moutain king/banshee queen's corpse. Or Tirion's.

What? Tons of characters in war3 with unique models translated into ordinary character models in WoW - namely, pretty much all the faction leaders. What does "Unique model" even mean in warcraft 3 - that the unit was a hero?

I would suggest that’s because the work on the instance (models, terrain etc) is started long before the lore of the instance is complete. When they start, the modeling team have a basic outline of the story and concept art to work from - i.e. a brief from creative along the lines of "you guys need to create a Bone Guard, Blood Queen, Scientist and a Minister of Death - here's some concept art of what we think."

All of the minor details (names, emotes etc) would have all been finished much later in the development cycle when creative finish the story.