There are a few reason when ICS are not the best option such as:
- Certain wonders buff specific tiles for one city, meaning you want to maximize the number of tiles the city can work
- Unit production works alot better with few powerful cities, military districts are stronger the more production a city have
There’s almost no circumstance you don’t want to settle cities 3 tiles apart. Usually it’s if coast or mountains or something gets in the way and doing a 4 tile space makes more sense.
The example you give of something like Ruhr valley or Petra just means you MIGHT want to swap some tiles and possibly do 4 tiles away in a specific instance. And there are only a handful of those instances in the game.
But ICS is unique from “go wide/expand/conquer.” The original motivation for the strategy was that city tiles were your best tiles, therefore you should have the largest % of your tiles being city centers as possible.
You can envision the difference between civ5 wide play- control a lot of tiles- and civ6’s optimal play: fit as many cities as possible into a given space. This “carpet of cities” is the heart of ICS and it absolutely is the best way to play civ6 for pretty much any victory. Only because we are humans that don’t want to click endlessly do we restrain ourselves - adding a city, no matter how crappy, is universally a positive in this game.
I find that once I establish my carpet of cities, it can make sense to add farms in because it raises my district capacity.
Yes, you can only build 1 of each district in a city, but almost every district adds value to your empire. It’s very cheap to spend 3 builder charges to add a farm triangle somewhere (Builders let you transfer production from one city to another- critical!) and that will give you a district slot or two.
Pretty much any city could use another district. You need farms (or fishing boats) to get to the point where you run out of decent districts to build. (Size 16ish.)
For you attempted min maxers who want everything to be size 10 while letting your people subsist on tundra shrubbery, farms also let you hit a target pop faster, and time is everything in Civ.