You guys are forgetting an important aspect of this. If you are playing a very militant game (like, if you're Rome) and you are following a hammer strategy - then most of your cities are going to be high production cities, with a lot of workshops and watermills. This synergizes with State Property and Caste System, allowing your cities to pump out military units. Now, instead of building cottages and trade cities, you can simply build a ton of hammer cities and a few specialist cities. When not at war you can make your hammer cities produce science. I know, I know, on a per city basis it won't compete with a trade city, but if you are twice the size of your next rival, you will stay ahead in tech. Also, it frees you to use the luxury slider and put it however high you want it, generating 15 extra happy faces to deal with emancipation (so that you can keep running CS). I recently played a game as Rome, with Julius Ceasar and did just that. I had a few specialist cities, with CS, and combined with the forum I had a lot of GPs, but most of my cities (about 35 or so) were production cities. I wiped my enemies off the standard Terra map quickly and I was surprised that I kept up in tech for the whole game, actually gaining a tech lead towards the very end.