Additionally, the Romans had organizational advantages in an advanced legal and commercial code. The Romans were also extremely flexible and combined conquered local traditions with their own (Roman paganism was particularly tolerant) and by expanding Roman citizenship in ever broader circles.
This is a great example of where the mindset of the ancient civilization is just so dramatically different than our own that it's difficult to understand their motivations.
Edit: to clarify, slaves are necessary to export culture because someone has to transcribe all of the books and laws etc.
Keep in mind that slavery for these people was nothing like the chattel slavery of the US south. Being an educated slave did mean that you had some restrictions on what you could do; it also freed you from having to worry about food and shelter, say. Similar tradeoffs have been made by salaried workers in Japan in the 70s and throughout the Western world during the industrial revolution
And even the restrictions part was ... varied. If you're a slave and your owner lives in Athens while you live in a Greek city on the Bosporus and supervise your owners business interests in the Black Sea (an actual example I recall from some of the primary sources from my Greek history classes in college), you really don't have much in the way of restrictions on you. Yes, legally you're a slave. So what? It has very little impact on your day-to-day life, at least as long as business is good.
I just don't think you can ever be totally secure psychologically while you own slaves, but as I said elsewhere in this thread, there are a lot of things about the Roman mindset that are very difficult for the modern mind to identify with, so at best, this is marginally informed speculation on my part.
