Much as Caste System seems good on paper, I've only once found a game where I had much use for it. A city that's a great people farm usually has enough improvements and wonders to have as many as is sensible of any particular specialist type. The only other one was where I was struggling to reach a cultural victory inside the time limit, so I switched in order to piles 10+ artists into my two great people factories to get some quick great artists to boost the first city to legendary point. That said, the other civics before emancipation aren't very enticing either. I tend to go for slavery as it's cheaper though, and pop-rushing is occasionally useful.