I agree with Zentuit. Enslaving is probably only useful in the beginning, when your starting cities are really small, and you want to grow them. But towards mid-game, when you're up to your neck in ...squalor, the last thing you want to do is to further increase your populace in your huge cities...
And since you have no control over how the slaves are distributed over your cities, it's probably better to just exterminate them. In the long run, it's better to have the small, slowly developing cities on your borders, with good civilized roman citizens, than to spend fortunes in trying to keep them from revolting all the time, tying lots of troops there, etc.
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