While it would be nice to do that, the thing to consider here is if that amount of detail could be customised on everything, the game would start to become stupidly complex.
A simpler way would just be for the game to choose the rarity based on the ship's stats, where more powerful ships appear less regularly. This would sort out the issue without giving the player more hassle to deal with.
I suppose You've got a point, but personally, I'd be fine with the extra complexity.
But You know what?
Both ideas could work at the same time. How about this:
By default, the game does exactly as You say. Without player interaction it assigns a global rarity to a ship based on it's potency, so the player would never
have to do it themselves if they don't want to.
... But the option to set the rarity manually is still there, if the player so desires.
(NOTE: I'm currently envisioning the rarity scale as a 1-to-10 slider. 1 being common; "You get this ship free in a box of cereal", up to 10, being rare; "You've got a better chance of seeing Bigfoot then one of these". )
Example A:
JohnDoe just made Himself a big ol' deathblock. He wants to put that baby in the pirate use catalog so he can fight it, along with the Isanth IV's, those TIE fighters He downloaded, and the Huge-assed Munchkintech capital ship He just got. He doesn't care about the hows and whys, He's just happy to throw them in there and see what happens. The game would automatically assign each of those ships a place on the rarity scale, without John's interaction. Johns happy, the game's happy, Bingo-bango.
Example B:
JoeDoe Is John's bro. Joe's also a severe nerd who stays up late thinking about who would win in a Star Wars V Star Trek fight. He's all about the nuts and bolts of the game. Joe just spent a few days building a light corvette, A replica of the ARC-170 fighter from Star wars, and downloaded the Munchkintech small fighter series. He has a whole system planed for his SP universe. After setting the ships as AI usable, He then clicks the rarity tab.
The ARC-170 is an old design, it should be uncommon in use, so Joe moves the rarity slider from it's computer-set default of
3 (common) up to
7 (Very uncommon), a number He personally feels good with.
The corvette was made by Joe as the Trading guild's primary transport in His game, so regardless of it's size and power, Joe slides it rarity from the computer-set default of
6, down to
3. Now the trading guild will heavily rely on His corvette.
He checks the rarity settings for the Munchkintech fighters, and decides they are fine where they are.
Joe's Happy, The game's happy.
I hate to make yet another Minecraft reference, but look at it like this:
In Minecraft You've got Red stone. The amount of highly complex things you can do with that stuff is mind-boggling. I've seen videos of some dude who made a working macro-sized version of an 8-bit CPU in Minecraft using Red stone switches and such.
Then You've guys like Me, who don't touch the shit, save for using a little bit of the stuff to rig up two iron doors to one button.
My simple interests in the stuff doesn't stop the mensa-rocket-surgeon from making works of industrial art, and their frighteningly over-complex usage of Red stone doesn't stop Me from enjoying the game My way.
TL;DR: Both options could work 100%, and never interfere with one another.
