Somehow, it seems to have slipped my notice that someone asked about letting other civs found religions and then just taking them.
This is certainly the most-likely circumstance, and happens all the time, but IME it doesn't really grab me. If you don't control the world's biggest religion(s), and you take the Shrine city (really sweet when they spent the Prophet so you don't have to), yea, you'll get a noticeable income boost, maybe enough to move your slider 10%. A lot of the time, though, these cities are not in handy locations, and worse, it's VERY unlikely you're going to have multiple religions founded in the same city.
When you have 2 or more religions in a city, and those Shrines are built, when you get Wall Street, you essentially multiply the amount of money you get by the number of Shrines there! If you have 50 cities in the world, would you rather have a bonus of 50 gpt from one religion or 250 gpt from five?
If you found the religion, you can hold off on founding new cities, so that they appear in the same city, and then focus your building on maximizing your income by working on only one city. Settle Merchants and Prophets there, build the Gold multiplier buildings, etc. Much cheaper to have to improve one city than all of them.
It does require an initial investment, though. It would probably be suicidal to restrict yourself to a single city in a high-difficulty game, and even to try to found an early religion if you had a competitor, and so you have no choice but to try to take them from other players.
). Number of turns is proportional on different game speed to achieved result. Well, some prefer Quick, some Marathon... 
