Caste System really is the key to most of your GPP whoring needs. In a typical game you may have a use for one, maybe two prophets to found shrines, but merchants and scientists are valuable to any strategy... Whereas artists are key to a culture win. With that civic in place you can actually tune a high food city to consistently generate the GP you want. As opposed to using wonderspam in a GP farm where you will usually wind up with quite a mixed bag in terms of what Great People you're getting... This may not be a big deal in the early game, but as the GPs get harder and harder to come by, it can cost you big time if you get the wrong type. This is why usually my capital will provide my first few great people thanks to a wonder or two that I've built there, but I will be busy establishing a food-rich city elsewhere that will be set up to generate GPs with massive amounts of caste-system enabled specialists. The beauty of those core 3 types of specialists and caste system is you can keep adding more specialists as the city grows (and assuming it has enough food) so that as the total GPPoints needed to get the next GP increases, so too does the amount of points you're generating. The capital, meanwhile, will occasionally give you an odd type GP just through rounding issues and whatnot, but late in the game you are probably only really after one type of great person... Be it merchant (ideal in warmonger games to mass upgrade troops), scientist (useful in most games types, absolutely clutch for space victories) or artists (for those culture bombs).
As for prophets well like I said, other than founding a shrine or two you probably don't really need them. They do, however, serve as a nice general purpose unit to settle in a city as they will give it an extra hammer and some gold... Maybe some science if you're under representation. I don't get too upset if I get extra great prophets from my oracle, I just settle them in my production or commerce cities, depending on what needs the boost. The real goal though is lots of settled scientists or lots of merchants to go on trade missions. Engineers can be nice for wonder spamming I guess, but really you don't need to build too many wonders over the course of the game, so I don't worry about them too much, although the one you typically get from the pyramids can be used to slingshot yourself into an oracle slingshot, or saved for the great library. All in all, after the BC years I'd prefer to not be getting engineers though, as cash, science or culture are far more valuable.
