I read your post as talking about enemy leveling, not enemy rarity. I just reread and see that you did use the word "rarity," but it still seems like you were talking about the same enemies being of varying difficulties. At least, that's my impression. Perhaps I read it wrong.
Rarity was a single cog in the leveling system I proposed. I think it is important for you to still run into rats at high level or trolls at low level but the chances of them spawning should vary depending on your level. When I was talking about the leveling part I was adding that while their encounter chances vary their strength should as well to a point, this extends the period during which the enemy is still a challenge while making it easier on low level characters and harder for high level characters. If you have enemies that level within a certain range then you can have them be more powerful than you at low level but not impossible and easy at higher levels but not pushovers.
Suppose you take a goblin, a relatively low level monster. If you have them at a static level (lets say 10) you can imagine them taking 10 hits to kill for a level 1 character but 1 hit to kill for a level 30 character. If you give them a sliding level within a limited range as per my argument (say 5-15) you can imagine them taking 7 hits to kill at when you are level 1, 2-3 hits to kill at level 30, and about 5 hits to kill at level 5-15. This makes it so they are not overpowering when you encounter them at low levels, not something to shrug off when you get marginally more powerful (they'd still end up turning into a 1 hit kill down the road), and makes them a steady challenge for a longer period of time. It increases accessibility of the game, decreases the strength of power leveling, but keeps the feeling of progress alive. Inputting the rarity mechanic into this example would mean that the goblin was common for a longer period but once you get x2 and above their max level they would begin becoming far less frequent while stronger monsters would become more frequent at their lowest levels.
EDIT: You can also add "encounter bonuses". Say there are 3 goblins and a goblin chieftain, you can make it so that the chieftain is always +2 levels above the goblins or other "less powerful creatures" around it up to its cap. So supposing the chieftain has a level range of 10-20, if you are level 10 the goblins would all be at their level 10 difficulty and the chieftain would be at the level 12 difficulty. If you are level 15 it'd be 15,17. And at 20 it'd be 15,20.