...When I started playing the game many years ago the reason I used that command was basically to see places I could go explore. So I wouldn't put it 1-30 (unless those NPCs were aggro) in the merchant example and I wouldn't put it as All in the potion example because neither of those fits that use case.

The other use cases of "there's a useful item or vendor here at my level" I don't think is something that would ever really work (everywhere would be 'All'). So the most reasonable bracket would be the levels you could explore that area at and have a reasonable chance of survival.

It might be something where a poll on facebook would make sense - what would you expect the level bracket on the areas command to show? As it is right now it's not very useful except for identifying cities and ultra-safe places (all the ones that say 'All' usually).