So I have often wondered why some demons and creatures that have great magical items don't really use them. I mean I understand why a bear-like animal wouldn't put on armor or use a shield but I love the idea that some demon at some point found a powerful item and decides to try it against you.
Just imagine plowing through Fallen and their cudgels and then there is one little fallen demon with a shiny sword and it swings at you and stuns your character for a moment. You think to yourself "How'd that little guy get that?" and decide to interrupt your speedrun and take him on to see what kind of magic item he really has. Naturally the better the item the harder the fight, but in turn, the better the reward.
Side Fantasy: I love the visual of a little Fallen trying out a magical wand that can cast meteors or something and starts to lose control. Meteors starting to rain down from the heavens wreaking havoc on both friend and foe.
I do think that this would be better than elite packs as a concept, but there is no reason they couldn't coexist or even coincide/intersect at times.
Also, I think there are other ways to get players to pay attention and get players to enjoy the details that the developers work so hard to put in. So let's say that instead of a monster deciding to put on or wield an item they decide to hoard it. Now, this idea is definitely in the Diablo series already but the behavior could be different. So as far as I can think the behavior of anything in diablo is to run toward you and attack. Some will run back and forth a little in a kiting pattern but that is about it. Going back to hoarding. Imagine a tribe of creatures that find an amulet and realize that it is magical or at least important to some degree, but they have no idea how to use it. Instead of trying to figure out what it does they hide it where they can keep it from those who would take it from them. So depending on the foe they'd hide it debris where no one would suspect or in their clan chest or down a secret passage. Now when a player appears some of them would be more defensive, not willing to go too far from their treasure; a leashing effect of sorts.
So imagine, as a player, going through one of the levels of a cathedral and most of the monsters do their standard attack on sight thing. Then you pass a room where you see that some do go and attack you and then there are some that are standing their ground making sure they stay between you and something else. Something important?. That is when you interrupt your speedrun to investigate what is so valuable to be guarded so closely.
Now, I know that including all the item visuals on different demons and monsters would be costly and time-intensive. Honestly, I'd even be happy just seeing the effects of a strong item rather than seeing both the item and the effect. Both ways are worth the investment for quality.
Conclusion: Letting some Demons/monsters use magical items has a great potential to add some fun and draw attention to detail. It can provide some interesting variety to what can turn into a grind. Also, monster behavior to guard things of great worth can give some variety to the actions of NPCs. My dream would be for Diablo 4 devs to see and implement this.