Something else being overlooked here is that Germany will have more diversity of districts than other civs, Being able to have 3 districts in a city from size one, and 6 or 7 as they hit the mid-teens in size, Germany will have more of the districts that other civs skip because of the limits. Since each type of CS grants bonuses to different types of districts, civs other than Germany may elect to take the city and its territory since they only have 1 or 2 campuses, theater squares, etc. Germany won't have a theater square in every city, but they are more likely to have a few of them so the bonus will be compounded more. Similarly with religion, I find Germany to be the most practical civ to work on your religion with (save Russia) because they can afford to have a holy site in 1/4 to 1/2 of their cities, so religious city state bonuses compound more with Germany. So I find I'm less likely as Germany to conquer a city-state than with other civs.
Also note that it' is a flat bonus, not a percentage bonus, so it doesn't scale over the game. Increasing a warrior's strength from 20 to 27 has a much greater impact than increasing a mech infantry's strength from 90 to 97. So do it early if you feel the need to exercise the bonus.
I think it's more of a bonus like Genghis had in civ5; it's not so much about conquering city states as much as it is that you can declare war on someone who has multiple suzerein and roll over their units more effectively without having to worry about trying to flip them first, which is more difficult than it was in civ5. You could also use them for XP farming, but I personally don't find that to be a great use for city-states, maybe others' mileage varies.