Heh. And suddenly running NPCs becomes ten times harder!
My problem when running a fight is that the PCs spend hours with their own, single character. They know them inside and out. They know every spell, every feat and every item on their character sheet, and how it works. Even better, they know how best to use them.
I, on the other hand, have to do that for every single NPC. Except that instead of weeks and months of learning how the NPC works, I get a few minutes (hours if I have the time to do enough prep).
Using the core rules plus some approved others makes that challenging at higher levels, but not impossible. Doing it with hundreds and hundreds of spells, items, feats, abilities, classes, etc. suddenly makes it too hard for me.
I admire anyone who can deal with that well. I know I can't. In that situation, my NPCs don't use their abilities properly.
So, simply for the sake of a smooth, challenging game, I have to approve non-core stuff on an item-by-item basis, whether that be spells, classes, feats, items, whatever.
Maybe I'm just not quick-minded enough.