I play in all 3 cities just for 3 x the fun (as each take up so little time in my day, the way I play them).
The unplanned benefits of this are:
- Extra diamonds to spend as I see fit. Yes from the Spire, but best of all from the Magic Academy chests. If you get two 500 diamond chests in two different cities at once, that's pretty handy!
- In Events, if there's a particular building I like the look of (literally the aesthetic look) I have 3 x the chances to get it and be able to place it somewhere
- You can experiment with structuring your city slightly differently, or see what the other "race" is like (my first city is Elven, but one of my others is Human)
Re. different strategies for different cities: I liked the idea of this in theory, but ended up playing pretty similarly in all three because it's just the style I enjoy the most and find the most productive. I have joined FAs in my 2nd and 3rd cities to test what the experience is like, but stay without a Fellowship in my first city as I like to keep it for relaxation with no obligations.