What a great question. The issue is complex, because slavery in the ancient world was very different than slavery in the modern age. Here are a few things to consider:
1. Slaves were often times very educated and refined. Often they were war prisoners from the Greek East. And remember that Greece was a very developed civilization.
2. Because of point one, slaves could do very important tasks such as manage businesses, teach, serve as doctors, etc.
3. If a person was a slave of a powerful patron, that slave would be more powerful that most citizen! In this sense, there is even evidence that people went into slavery to move up the social ladder.
4. Manumission was very possible. Also slaves were often given money for their work. The Romans called this: peculium.
5. Manumitted slaves, at times, grew in great power. Suetonius' Claudius make mention of powerful freedmen!
Check out this insightful little essay on slavery in the Roman world:
For more bibliography, see:
The Roman Law of Slavery (Cambridge: Cambridge University Press, 1908); Alan Watson. Roman Slave Law (Baltimore: Johns Hopkins University Press, 1987); Ramsey MacMullen. “Late Roman Slavery,” Historia 36, 1987; K. R Bradley. Slaves and Masters in the Roman Empire (Oxford: Oxford University Press, 1987); K. R. Bradley.Slaves and Society at Rome (Cambridge: Cambridge University Press, 1994).
Slaves were considered living property in Rome. They were akin to cattle: livestock.
Even once freed, manumitted slaves could lose their freedom for fairly minor offenses. (Look up the lex Aelia Sentia for specific details on Augustus' actions here.)
A bit later, the lex Fufia Caninia limited how many slaves could be manumitted by a landholder upon his death.
Slaves could buy their freedom if they earned enough money through their labors, but could not become citizens.