For all clarity, let's forget about making the production of specific objects depend on such a system. It's purely abstract.
And just suddenly now you're free to purchase their guarded secrets for the price of a few socks?
No, you need good relations (allowing citizens to travel freely across the border), and transferring the skill requires the time to train a dwarf, and he would typically pay with his labor for the duration of the training. It's not a state secret, because all craftsmen in a fortress are supposed to have access to that knowledge.
And if you can't buy them, what's the point?
Buying doesn't magic knowledge from one place to the other, nor is it the kind of knowledge you can write down on a piece of parchment, like the formula of gunpowder. Transferring know-how requires the transfer of skilled people. To convince that master blacksmith to leave his succesful business over in the mountain halls to come to your hole in the ground, you should have to make an effort.
Forever stuck at sub par quality items because the RNG deemed it so.
They aren't sub par; they're the best that are available, and the happy thoughts should reflect that. Don't forget that items without modifiers are standard, anything above that is a plus.
If you're playing to max out the numbers that's fine, but why should that be guaranteed to be quick, easy and available without particular effort?
Ooh, maybe sneak the secret of sockmaking from the Vomits of Circling. How would you implement that? Would it add anything but tedium into a game if every world you'd have to start with an adventurer that learns the secrets of knitting so your fort can have access to it?
No, send a dwarf to live at the Vomits of Circling, with a secret mission to steal trade secrets. He'll return with the secrets after a few years if all goes well. Alternatively, improve your relations with the Vomits, or with the clothiers there, or attack them and capture one, etc.
I called it fake difficulty since I believe labors difficult enough to have secrets like that should feature apprenticeship by a master until proficient anyway which in turn teaches you all you need.
So a body of knowledge in a particular fortress is either universal and exhaustive, or completely absent?
You don't teach a novice by randomly picking stuff and showing it to him, kicking him out to a new settlement and hoping his new friends know the tricks you didn't teach.
He can learn everything his fortress knows about the craft, which isn't everything that could possibly known about it.
And "exceptional" doesn't necessarily imply you know all the tricks in the entire realm, it means you know the right tricks
If an item is of exceptional quality, that means that most items are worse.
The whole idea is to make dwarven civs and fortresses more diverse, and give them a local culture. It's a way to simulate the exchange and ideas and the spread of knowledge.